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Authors:S. Naqshband; A. J. F. Hoitink, B. McElroy, D. Hurther, S. J. M. H. HulscherAbstract: Sandy river beds are dominated by rhythmic features known as dunes. Experimental investigations of turbulent flow and sediment transport over dunes have predominantly focused on equilibrium flows that are rare in natural rivers. Using a novel acoustic instrument over migrating dunes in a laboratory setting, we quantify a number of dynamical properties that are crucial in our understanding and modeling of dune morphology and kinematics, particularly under nonequilibrium flows during dune transition to upper stage plane bed. Measured sediment transport distributions reveal a positive spatial lag between dune crest and maximum sediment transport rate that eventually caused washing out of dunes. Bed load was entirely captured in dune troughs, contributing to dune translation where most of suspended load was advected further downstream contributing to dune deformation. Measured bypass fraction was about 76%, which means that only 24% of the total sediment load at the dune crest contributed to dune migration.PubDate: 2017-11-23T19:06:36.156604-05:DOI: 10.1002/2017GL075906

Authors:Ruifen Zhan; Yuqing Wang, Jiuwei ZhaoAbstract: Intense tropical cyclones (TCs) formed over the western part of the western North Pacific (WWNP) pose greater coastal risk than those formed over the eastern part (EWNP). Here we show that the proportion of intense TCs relative to all TCs (PITC) locally formed over the WWNP west of 140°E has increased significantly by about 16%~20%, or equivalently almost doubled, since the late 1970s. However, the PITC over the EWNP has experienced little change. This sharp west-east contrast is primarily driven by the La Nina-like sea surface temperature (SST) pattern in the Pacific, identified as the mega-ENSO in previous studies. It is shown that the strong warming in the WWNP has contributed greatly to the increased PITC. In the EWNP, the dynamical conditions associated with the intensified mega-ENSO play a vital opposite role in offsetting and even overweighting the effect of local SST warming.PubDate: 2017-11-22T13:00:48.053263-05:DOI: 10.1002/2017GL075916

Authors:Zhongxiang ZhaoAbstract: The superposition of two waves of slightly different wavelengths has long been used to illustrate the distinction between phase velocity and group velocity. The first-mode M2 and S2 internal tides exemplify such a two-wave model in the natural ocean. The M2 and S2 tidal frequencies are 1.932 and 2 cycles per day, respectively, and their superposition forms a spring-neap cycle in the semidiurnal band. The spring-neap cycle acts like a wave, with its frequency, wavenumber and phase being the differences of the M2 and S2 internal tides. The spring-neap cycle and energy of the semidiurnal internal tide propagate at the group velocity. Long-range propagation of M2 and S2 internal tides in the North Pacific is observed by satellite altimetry. Along a 3400-km beam spanning 24° –54° N, the M2 and S2 travel times are 10.9 and 11.2 days, respectively. For comparison, it takes the spring-neap cycle 21.1 days to travel over this distance. Spatial maps of the M2 phase velocity, the S2 phase velocity, and the group velocity are determined from phase gradients of the corresponding satellite observed internal tide fields. The observed phase and group velocities agree with theoretical values estimated using the World Ocean Atlas 2013 (WOA13) annual-mean ocean stratification.PubDate: 2017-11-22T13:00:45.132375-05:DOI: 10.1002/2017GL076008

Authors:Patricia M. Lawston; Joseph A. Santanello, Sujay V. KumarAbstract: Irrigation can influence weather and climate, but the magnitude, timing, and spatial extent of irrigation are poorly represented in models, as are the resulting impacts of irrigation on the coupled land-atmosphere system. One way to improve irrigation representation in models is to assimilate soil moisture observations that reflect an irrigation signal to improve model states. Satellite remote sensing is a promising avenue for obtaining these needed observations on a routine basis, but to date, irrigation detection in passive microwave satellites has proven difficult. In this study, results show that the new Enhanced soil moisture product from the Soil Moisture Active Passive (SMAP) satellite is able to capture irrigation signals over three semi-arid regions in the western United States. This marks an advancement in earth-observing satellite skill and the ability to monitor human impacts on the water cycle.PubDate: 2017-11-22T13:00:30.955731-05:DOI: 10.1002/2017GL075733

Authors:Tongwen Li; Huanfeng Shen, Qiangqiang Yuan, Xuechen Zhang, Liangpei ZhangAbstract: Fusing satellite observations and station measurements to estimate ground-level PM2.5 is promising for monitoring PM2.5 pollution. A geo-intelligent approach, which incorporates geographical correlation into an intelligent deep learning architecture, is developed to estimate PM2.5. Specifically, it considers geographical distance and spatiotemporally correlated PM2.5 in a deep belief network (denoted as Geoi-DBN). Geoi-DBN can capture the essential features associated with PM2.5 from latent factors. It was trained and tested with data from China in 2015. The results show that Geoi-DBN performs significantly better than the traditional neural network. The out-of-sample cross-validation R2 increases from 0.42 to 0.88, and RMSE decreases from 29.96 to 13.03μg/m3. On the basis of the derived PM2.5 distribution, it is predicted that over 80% of the Chinese population live in areas with an annual mean PM2.5 of greater than 35μg/m3. This study provides a new perspective for air pollution monitoring in large geographic regions.PubDate: 2017-11-22T13:00:27.298317-05:DOI: 10.1002/2017GL075710

Authors:David J. Farnham; Scott Steinschneider, Upmanu LallAbstract: Seasonal precipitation forecasts over the contiguous United States (CONUS) during the 2015-16 El Niño exhibited significant bias over many regions, especially in the Western United States where seasonal information is particularly valuable for reservoir operation. Diagnosing the origin of this bias requires understanding the empirical signal from tropical heating to mid-latitude precipitation. In this paper, we find that atmospheric zonal wind indices computed over the region typically associated with the winter jet stream provide a skillful, spatially distributed, linear prediction of precipitation over CONUS, over all winters (Jan-Mar; JFM). Furthermore, we show that more (less) central (eastern) Pacific Ocean heating may have contributed to the unexpected 2016 JFM CONUS precipitation and that this was likely predictable based on antecedent (Dec) sea surface temperatures. The zonal wind indices act as intermediate variables in a causal chain and our analyses provide support for the potential for empirical prediction and also a diagnostic for physics based models to help improve forecasts.PubDate: 2017-11-21T18:40:36.523486-05:DOI: 10.1002/2017GL075959

Authors:Matthew H. Alford; Bernadette M. Sloyan, Harper L. SimmonsAbstract: Internal waves, which drive most ocean turbulence and add “noise" to lower-frequency records, interact with low-frequency current systems and topography in yet poorly known ways. Taking advantage of a heavily-instrumented, 14-month mooring array, internal waves in the East Australia Current (EAC) are examined for the first time. Internal wave horizontal kinetic energy (HKE) is within a factor of two of the Garrett-Munk (1976) spectrum. Continuum internal waves, near-inertial waves and internal tides together constitute a significant percentage of the total velocity variance. Mode-1 internal tide energy fluxes are southward and much smaller than energy times group velocity, consistent with reflection at the continental slope of incident waves generated from near New Caledonia and the Solomon Islands. Internal tide HKE is highly phase-variable, consistent with refraction by the variable EAC. Mode-1 near-inertial wave energy fluxes are of comparable magnitude and are equatorward and episodic, consistent with generation by storms farther poleward. These processes are considered together in the complex environment of the EAC.PubDate: 2017-11-21T18:40:32.894469-05:DOI: 10.1002/2017GL075246

Authors:D. Lin; B. Zhang, W. A. Scales, M. Wiltberger, C. R. Clauer, Z. XuAbstract: The role of solar wind density in the cross polar cap potential (CPCP) response under northward interplanetary magnetic field (IMF) is investigated with observation-based global simulations. A rare event was reported by Clauer et al. [2016] during which the ionospheric electric field EISP does not saturate under extreme interplanetary electric field (IEF) of ∼15 mV/m. While commonly utilized coupling functions based on IEF fail to provide an unambiguous explanation for the linear response, the LFM-MIX model is used to explore the mechanisms in this study. The model first reproduces the observed linear features of the EISP. The simulated CPCP also responds linearly to IEF variations. A controlled simulation is designed with solar wind density artificially reduced to 10% of the observed value while all other parameters such as the IEF are kept the same. The controlled simulation shows saturation of the EISP as well as the CPCP. Further analysis shows the difference in the magnetosheath plasma β, implying the distinct dominant forces between the two simulations. The Lopez magnetosheath force balance theory is used to explain the CPCP responses under different solar wind densities. This comparison study highlights the role of solar wind density in determining the magnetosphere-ionosphere response to extreme interplanetary drivings.PubDate: 2017-11-21T18:40:29.066167-05:DOI: 10.1002/2017GL075275

Authors:Nathaniel J. Lindsey; Eileen R. Martin, Douglas S. Dreger, Barry Freifeld, Steve Cole, Stephanie R. James, Biondo L. Biondi, Jonathan B. Ajo-FranklinAbstract: Our understanding of subsurface processes suffers from a profound observation bias: seismometers are sparse and clustered on continents. A new seismic recording approach, Distributed Acoustic Sensing (DAS), transforms telecommunication fiber-optic cables into sensor arrays enabling meter-scale recording over tens of kilometers of linear fiber length. We analyze cataloged earthquake observations from three DAS arrays with different horizontal geometries to demonstrate some possibilities using this technology. In Fairbanks, Alaska, we find that stacking ground motion records along 20 meters of fiber yields a waveform that shows a high degree of correlation in amplitude and phase with a co-located inertial seismometer record at 0.8 - 1.6 Hz. Using an L-shaped DAS array in Northern California, we record the nearly vertically-incident arrival of an earthquake from The Geysers Geothermal Field and estimate its backazimuth and slownesses via beamforming for different phases of the seismic wavefield. Lastly, we install a fiber in existing telecommunications conduits below Stanford University and show that little cable-to-soil coupling is required for teleseismic P- and S-phase arrival detection.PubDate: 2017-11-20T17:37:44.415963-05:DOI: 10.1002/2017GL075722

Authors:Shuang Yi; Kosuke Heki, An QianAbstract: Global mean sea level rise has been accelerating for more than 100 years, and the acceleration in the last two decades seems to further increase. The latest development in geodetic and marine observations enable us to scrutinize and understand the sources of the sea level acceleration in the last decade. For this end, observations from satellite altimetry, gravimetry, and in-situ measurements of the ocean between 2005 and 2015 are combined and their closure is examined. Our results show that the acceleration during the last decade (0.27 ± 0.17 mm/yr2) is about three times faster than its value during 1993–2014. The acceleration comes from three factors, i.e. 0.04 ± 0.01 mm/yr2 (~15%) by land ice melting, 0.12 ± 0.06 mm/yr2 (~44%) by thermal expansion of the sea water, and 0.11 ± 0.02 mm/yr2 (~41%) by declining land water storage. Although these values in 11 years may suffer from natural variabilities, they shed light on the underlying mechanisms of sea level acceleration and reflect its susceptibility to the global warming.PubDate: 2017-11-20T17:37:04.172458-05:DOI: 10.1002/2017GL076129

Authors:Jinxing Li; Jacob Bortnik, Xin An, Wen Li, Richard M. Thorne, Meng Zhou, William S. Kurth, George B. Hospodarsky, Herbert O. Funsten, Harlan E. SpenceAbstract: Using high-resolution waveforms measured by the Van Allen Probes, we report a novel observation in the radiation belts. Namely, we show that multiband, discrete, rising-tone whistler-mode chorus emissions exhibit a one-to-one correlation with Langmuir wave bursts. Moreover, the periodic Langmuir wave bursts are generally observed at the phase location where the chorus wave E component is oriented opposite to its propagation direction. The electron measurements show a beam in phase space density at the particle velocity that matches the parallel phase velocity of the chorus waves. Based on this evidence, we conclude that the chorus waves accelerate the suprathermal electrons via Landau resonance, and generate a localized electron beam in phase space density. Consequently, the Langmuir waves are excited locally and are modulated by the chorus wave phase. This microscale interaction between chorus waves and high frequency electrostatic waves provides a new insight into the nonlinear wave-particle interaction process.PubDate: 2017-11-20T17:36:39.62695-05:0DOI: 10.1002/2017GL075877

Authors:Zhiqiang Wang; Hao Zhai, Zhuxiu GaoAbstract: Hydrogen band electromagnetic ion cyclotron (EMIC) waves have received much attention recently because they are found to frequently span larger spatial areas than the other band EMIC waves. Using test particle simulations, we study the nonlinear effects of hydrogen band EMIC waves on ring current H+ ions. A dimensionless parameter R is used to characterize the competition between wave-induced and adiabatic motions. The results indicate that there are three regimes of wave-particle interactions for typical 35 keV H+ ions at L=5: diffusive (quasi-linear) behavior when αeq ≤ 35° (R ≥ 2.45), the nonlinear phase trapping when 35° < αeq < 50° (0.75 < R < 2.45), and both the nonlinear phase bunching and phase trapping when αeq ≥ 50° (R ≤ 0.75). The phase trapping can transport H+ ions towards large pitch angle, while the phase bunching has the opposite effect. The phase trapped H+ ions can be significantly accelerated (from 35 keV to over 500 keV) in about 4 minutes, and thus contribute to the formation of high energy components of ring current ions. The results suggest that the effect of hydrogen band EMIC waves is not ignorable in the nonlinear acceleration and resonance scattering of ring current H+ ions.PubDate: 2017-11-20T17:35:28.997896-05:DOI: 10.1002/2017GL075843

Authors:Jennifer L. Miselis; Jorge Lorenzo-TruebaAbstract: Storm-driven sediment fluxes onto and behind barrier islands help coastal barrier systems keep pace with sea-level rise (SLR). Understanding what controls cross-shore sediment flux magnitudes is critical for making accurate forecasts of barrier response to increased SLR rates. Here, using an existing morphodynamic model for barrier island evolution, observations are used to constrain model parameters and explore potential variability in future barrier behavior. Using modeled drowning outcomes as a proxy for vulnerability to SLR, 0%, 28%, and 100% of the barrier is vulnerable to SLR rates of 4, 7 and 10mm/y, respectively. When only overwash fluxes are increased in the model, drowning vulnerability increases for the same rates of SLR, suggesting future increases in storminess may increase island vulnerability particularly where sediment resources are limited. Developed sites are more vulnerable to SLR, indicating anthropogenic changes to overwash fluxes and estuary depths could profoundly affect future barrier response to SLR.PubDate: 2017-11-20T17:35:26.258836-05:DOI: 10.1002/2017GL074811

Authors:Anthea J. Coster; Larisa Goncharenko, Shun-Rong Zhang, Philip J. Erickson, William Rideout, Juha VierinenAbstract: On 21 August 2017, during daytime hours, a total solar eclipse with a narrow ∼160 km wide umbral shadow occurred across the continental US. Totality was observed from the Oregon coast at ∼9:15 LST (17:20 UT) to the South Carolina coast at ∼13:27 LST (18:47 UT). A dense network of GNSS receivers was utilized to produce total electron content (TEC) and differential TEC. These data were analyzed for the latitudinal and longitudinal response of the TEC and for the presence of traveling ionospheric disturbances (TIDs) during eclipse passage. A significant TEC depletion, in some cases greater than 60%, was observed associated with the eclipse shadow, exceeding initial model predictions of 35% [Huba and Drob, 2017]. Evidence of enhanced large scale TID activity was detected over the US prior to and following the large TEC depletion observed near the time of totality. Signatures of enhanced TEC structures were observed over the Rocky Mountain chain during the main period of TEC depletion.PubDate: 2017-11-20T17:35:24.030616-05:DOI: 10.1002/2017GL075774

Authors:James O'Donoghue; Luke Moore, John E. P. Connerney, Henrik Melin, Tom S. Stallard, Steve Miller, Kevin H. BainesAbstract: In April 2011 Saturn's mid-latitude ionospheric 3+ emissions were detected, exhibiting anomalous (non-solar) 3+ latitudinal variations consistent with the transport of water from specific locations in Saturn's rings, known as ‘ring rain’. These products, transported to the plan et along the magnetic field, may help to explain the unusual pattern of peaks and troughs in electron densities discovered in Saturn's ionosphere by spacecraft flybys. In the present study, we analyzed 3+ emissions recorded on 23 April 2013, showing for the first time since the original detection that Saturn's mid latitude 3+ emissions are indeed heavily modified. Although the 2013 emissions are dimmer by almost a factor of 3.7, the latitudinal contrast is greater and uncertainties are lower. Increased 3+ intensities were found near planetocentric latitudes of 43° , 51° and 63° , previously identified with sources at the inner edge of the B ring, A ring, and the orbit of Enceladus and associated E-ring.PubDate: 2017-11-20T17:34:46.492244-05:DOI: 10.1002/2017GL075932

Authors:Samuel M. Haugland; Jeroen Ritsema, Satoshi Kaneshima, Michael S. ThorneAbstract: Broadband USArray recordings of the July 21, 2007 western Brazil earthquake (MW=6.0; depth = 633 km) include high-amplitude signals about 40 s, 75 s, and 100 s after the P wave arrival. They are consistent with S-wave to P-wave conversions in the mantle beneath northwestern South America. The signal at 100 s, denoted as S1750P, has the highest amplitude and is formed at 1750 km depth based on slant-stacking and semblance analysis. Waveform modeling using axisymmetric, finite-difference synthetics indicates that S1750P is generated by a 10-km thick heterogeneity, presumably a fragment of subducted mid-ocean ridge basalt in the lower mantle. The negative polarity of S1750P is a robust observation and constrains the shear-velocity anomaly δVS of the heterogeneity to be negative. The amplitude of S1750P indicates that δVS is in the range from -1.6% to -12.4%. The large uncertainty in δVS is due the large variability in the recorded S1750P amplitude and simplifications in the modeling of S1750P waveforms. The lower end of our estimate for δVS is consistent with ab initio calculations by Tsuchiya [2011], who estimated that δVS of eclogite at lower-mantle pressure is between 0 and -2% due to shear softening from the post-stishovite phase transition.PubDate: 2017-11-20T17:33:40.955263-05:DOI: 10.1002/2017GL075463

Authors:A. G. Demekhov; J. Manninen, O. Santolik, E. E. TitovaAbstract: We present results of simultaneous observations of VLF chorus elements at the ground based station Kannuslehto in Northern Finland and on board Van Allen Probe A. Visual inspection and correlation analysis of the data reveal one-to-one correspondence of several (at least 12) chorus elements following each other in a sequence. Poynting flux calculated from electromagnetic fields measured by the EMFISIS instrument on board Van Allen Probe A shows that the waves propagate at small angles to the geomagnetic field and oppositely to its direction, i.e., from Northern to Southern geographic hemisphere. The spacecraft was located at L≃4.1 at a geomagnetic latitude of −12.4∘ close to the plasmapause and inside a localized density inhomogeneity with about 30% density increase and a transverse size of about 600 km. The time delay between the waves detected on the ground and on the spacecraft is about 1.3 s, with ground-based detection leading spacecraft detection. The measured time delay is consistent with the wave travel time of quasi-parallel whistler mode waves for a realistic profile of the plasma density distribution along the field line. The results suggest that chorus discrete elements can preserve their spectral shape during a hop from the generation region to the ground followed by reflection from the ionosphere and return to the near-equatorial region.PubDate: 2017-11-20T17:33:19.98436-05:0DOI: 10.1002/2017GL076139

Authors:A. L. Jones; D. R. Feldman, S. Freidenreich, D. Paynter, V. Ramaswamy, W. D. Collins, R. PincusAbstract: A new paradigm in benchmark absorption-scattering radiative transfer is presented that enables both the globally-averaged and spatially-resolved testing of climate model radiation parameterizations in order to uncover persistent sources of biases in the aerosol Instantaneous Radiative Effect (IRE). A proof-of-concept is demonstrated with the GFDL AM4 and CESM 1.2.2 climate models. Instead of prescribing atmospheric conditions and aerosols, as in prior intercomparisons, native snapshots of the atmospheric state and aerosol optical properties from the participating models are used as inputs to an accurate radiation solver to uncover model-relevant biases. These diagnostic results show that the models’ aerosol IRE bias is of the same magnitude as the persistent range cited (~1 W/m2), and also varies spatially and with intrinsic aerosol optical properties. The findings underscore the significance of native model error analysis and its dispositive ability to diagnose global biases, confirming its fundamental value for the Radiative Forcing Model Intercomparison Project.PubDate: 2017-11-20T17:33:16.018397-05:DOI: 10.1002/2017GL075933

Authors:James F. Booth; Etienne Dunn-Sigouin, Stephan PfahlAbstract: The path and speed of extratropical cyclones along the east coast of North America influence their societal impact. This work characterizes the climatological relationship between cyclone track path and speed, and blocking and the North Atlantic Oscillation (NAO). An analysis of Lagrangian cyclone track propagation speed and angle shows that the percentage of cyclones with blocks is larger for cyclones that propagate northward or southeastward, as is the size of the blocked region near the cyclone. Cyclone-centered composites show propagation of cyclones relative to blocks is consistent with steering by the block: northward tracks more often have a block east/northeast of the cyclone; slow tracks tend to have blocks due north of the cyclone. Comparison with the NAO shows that to first order blocking and the NAO steer cyclones in a similar manner. However, blocked cyclones are more likely to propagate northward, increasing the likelihood of cyclone related impacts.PubDate: 2017-11-20T17:33:12.575396-05:DOI: 10.1002/2017GL075941

Authors:Thomas Frederikse; Riccardo E. M. Riva, Matt A. KingAbstract: Present-day mass redistribution increases the total ocean mass and, on average, causes the ocean bottom to subside elastically. Therefore, barystatic sea-level rise is larger than the resulting global-mean geocentric sea-level rise, observed by satellite altimetry and GPS-corrected tide gauges. We use realistic estimates of mass redistribution from ice-mass loss and land-water storage to quantify the resulting ocean-bottom deformation and its effect on global and regional ocean-volume change estimates. Over 1993-2014, the resulting globally-averaged geocentric sea-level change is 8 percent smaller than the barystatic contribution. Over the altimetry domain, the difference is about 5 percent, and due to this effect, barystatic sea-level rise will be underestimated by more than 0.1 mm/y over 1993-2014. Regional differences are often larger: up to 1 mm/y over the Arctic Ocean and 0.4 mm/y in the South Pacific. Ocean bottom deformation should be considered when regional sea-level changes are observed in a geocentric reference frame.PubDate: 2017-11-17T17:50:40.120472-05:DOI: 10.1002/2017GL075419

Authors:Qi Liu; Yonghong Hao, Elaine Stebler, Nobuaki Tanaka, Chris B. ZouAbstract: Mapping the spatiotemporal patterns of soil moisture within heterogeneous landscapes is important for resource management and for the understanding of hydrological processes. A critical challenge in this mapping is comparing remotely-sensed or in-situ observations from areas with different vegetation cover but subject to the same precipitation regime. We address this challenge by wavelet analysis of multi-year observations of soil moisture profiles from adjacent areas with contrasting plant functional types (grassland, woodland, and encroached) and precipitation. The analysis reveals the differing soil moisture patterns and dynamics between plant functional types. The coherence at high-frequency periodicities between precipitation and soil moisture generally decreases with depth but this is much more pronounced under woodland compared to grassland. Wavelet analysis provides new insights on soil moisture dynamics across plant functional types and is useful for assessing differences and similarities in landscapes with heterogeneous vegetation cover.PubDate: 2017-11-17T17:50:37.625866-05:DOI: 10.1002/2017GL075542

Authors:Soroush Rezvanbehbahani; Leigh A. Stearns, Amir Kadivar, J. Douglas Walker, C. J. van der VeenAbstract: Geothermal heat flux (GHF) is a crucial boundary condition for making accurate predictions of ice sheet mass loss, yet it is poorly known in Greenland due to inaccessibility of the bedrock. Here we use a machine learning algorithm on a large collection of relevant geologic features and global GHF measurements, and produce a GHF map in Greenland which we argue is within ∼15% accuracy. The main features of our predicted GHF map include a large region with high GHF in central-north Greenland surrounding the NorthGRIP ice core site, and hotspots in the Jakobshavn Isbræ catchment, upstream of Petermann Gletscher, and near the terminus of Nioghalvfjerdsfjorden glacier. Our model also captures the trajectory of Greenland movement over the Icelandic plume by predicting a stripe of elevated GHF in central-east Greenland. Finally, we show that our model can produce substantially more accurate predictions if additional measurements of GHF in Greenland are provided.PubDate: 2017-11-16T17:50:44.018631-05:DOI: 10.1002/2017GL075661

Authors:Brendan J. Meade; Phoebe M. R. DeVries, Jeremy Faller, Fernanda Viegas, Martin WattenbergAbstract: Aftershocks may be triggered by the stresses generated by preceding mainshocks. The temporal frequency and maximum size of aftershocks are well described by the empirical Omori and Bath laws, but spatial patterns are more difficult to forecast. Coulomb failure stress is perhaps the most common criterion invoked to explain spatial distributions of aftershocks. Here we consider the spatial relationship between patterns of aftershocks and a comprehensive list of 38 static elastic scalar metrics of stress (including stress tensor invariants, maximum shear stress, and Coulomb failure stress) from 213 coseismic slip distributions worldwide. The rates of true-positive and false-positive classification of regions with and without aftershocks are assessed with receiver operating characteristic (ROC) analysis. We infer that the stress metrics that are most consistent with observed aftershock locations are maximum shear stress and the magnitude of the second and third invariants of the stress tensor. These metrics are significantly better than random assignment at a significance level of 0.005 in over 80% of the slip distributions. In contrast, the widely-used Coulomb failure stress criterion is distinguishable from random assignment in only 51-64% of the slip distributions. These results suggest that a number of alternative scalar metrics are better predictors of aftershock locations than classic Coulomb failure stress change.PubDate: 2017-11-16T17:50:32.738725-05:DOI: 10.1002/2017GL075875

Authors:Christopher P. Konrad; Michael D. DettingerAbstract: Atmospheric rivers (AR) have a significant role in generating floods across the western United States. We analyze daily streamflow for water years 1949 to 2015 from 5477 gages in relation to water vapor transport by ARs using a 6-hour chronology resolved to 2.5° latitude and longitude. The probability that an AR will generate 50 mm/day of runoff in a river on the Pacific Coast increases from 12% when daily mean water vapor transport, DVT, is greater than 300 kg m-1s-1 to 54% when DVT>600 kg m-1s-1. Extreme runoff, represented by the 99th quantile of daily values, doubles from 80 mm/day at DVT=300 kg m-1s-1 to 160 mm/day at DVT=500 kg m-1s-1. Forecasts and predictions of water vapor transport by atmospheric rivers can support flood risk assessment and estimates of future flood frequencies and magnitude in the western United States.PubDate: 2017-11-15T19:55:45.721365-05:DOI: 10.1002/2017GL075399

Authors:Benjamin Bronselaer; Michael Winton, Joellen Russell, Christopher L. Sabine, Samar KhatiwalaAbstract: Previous studies found large biases between individual observational and model estimates of historical ocean anthropogenic carbon uptake. We show that the largest bias between the Coupled Model Intercomparison Project phase 5 (CMIP5) ensemble mean and between two observational estimates of ocean anthropogenic carbon is due to a difference in start date. After adjusting the CMIP5 and observational estimates to the 1791-1995 period, all three carbon uptake estimates agree to within 3 Pg of C, about 4% of the total. The CMIP5 ensemble mean spatial bias compared to the observations is generally smaller than the observational error, apart from a negative bias in the Southern Ocean, and a positive bias in the Southern Indian and Pacific Oceans compensating each other in the global mean. This dipole pattern is likely due to an equatorward and weak bias in the position of Southern Hemisphere westerlies and lack of mode and intermediate water ventilation.PubDate: 2017-11-15T19:55:43.71085-05:0DOI: 10.1002/2017GL074435

Authors:V. Bruno; M. Mattia, E. Montgomery-Brown, M. Rossi, D. ScanduraAbstract: Global Positioning System (CGPS) data from Mt. Etna between May 2015 and September 2016 show intense inflation and a concurrent Slow Slip Event (SSE) from 11 December 2015 to 17 May 2016. In May 2016, an eruptive phase started from the summit craters, temporarily stopping the ongoing inflation. The CGPS data presented here give us the opportunity to determine 1) the source of the inflating body; 2) the strain rate parameters highlighting shear strain rate accumulating along NE Rift and S Rift; 3) the magnitude of the SSE; and 4) possible interaction between modelled sources and other flank structures through stress calculations.By analytical inversion, we find an inflating source 5.5 km under the summit (4.4 km b.s.l) and flank slip in a fragmented shallow structure accommodating displacements equivalent to a magnitude Mw 6.1 earthquake. These large displacements reflect a complex mechanism of rotations indicated by the inversion of CGPS data for strain rate parameters. At the scale of the volcano, these processes can be considered precursors of seismic activity in the eastern flank of the volcano, but concentrated mainly on the northern boundary of the mobile eastern flank along the Pernicana Fault and in the area of the Timpe Fault System.PubDate: 2017-11-15T19:55:38.687454-05:DOI: 10.1002/2017GL075744

Authors:A. Grocott; H. J. Laurens, J. A. WildAbstract: We present SuperDARN radar observations of ionospheric convection during substorms. Substorms were grouped according to their onset latitude, onset magnetic local time, and the prevailing sense of IMF BY. The radar observations were then sorted according to substorm group and average convection patterns produced. Here, we discuss the patterns corresponding to substorms with onsets occurring in the 65∘−67∘ onset latitude range, at either early (20 − 22h) or late (01 − 03h) magnetic local times, during intervals of either dominant positive or negative IMF BY. We show that the morphology of the convection patterns differs from that predicted by existing empirical models, with the location of the nightside convection throat being largely consistent with the location of substorm onset. The expected IMF BY-induced dawn-dusk convection asymmetry can be enhanced on the nightside when the substorm onset occurs at a fortuitous location, but can equally be removed or even reversed from this expected state. Thus the nightside convection asymmetries are seemingly unrelated to the instantaneous sense of IMF BY.PubDate: 2017-11-15T19:55:34.272253-05:DOI: 10.1002/2017GL075763

Authors:Lin Guo; Peng Xiu, Fei Chai, Huijie Xue, Dongxiao Wang, Jun SunAbstract: New evidences were provided that Kuroshio intrusion in winter is able to increase phytoplankton growth in the open ocean of the northern South China Sea (SCS) based on multiple data sources. Strong fronts due to Kuroshio intrusion and interactions with the SCS water are associated with intense upwelling, supplying high nutrients from the subsurface SCS water and increasing phytoplankton productivity in the frontal region. High chlorophyll is more dynamically related to these fronts than to the alongshore wind, wind stress curl and eddy kinetic energy on interannual timescale. Further examinations suggest fronts associated with Kuroshio intrusion into the SCS are linked with large-scale climate variability. During El Niño years, stronger Kuroshio intrusion results in stronger fronts that generate intensified local upwelling and enhanced Luzon winter blooms (LZBs).PubDate: 2017-11-15T19:55:29.471965-05:DOI: 10.1002/2017GL075336

Authors:Brandon P. VanderBeek; Douglas R. ToomeyAbstract: The anisotropic fabric of the oceanic mantle lithosphere is often assumed to parallel paleo-relative plate motion (RPM). However, we find evidence that this assumption is invalid beneath the Juan de Fuca (JdF) plate. Using travel-times of seismic energy propagating through the topmost mantle we find that the fast direction of P-wave propagation is rotated 18° ± 3° counterclockwise to the paleo-spreading direction and strikes between Pacific-JdF relative and JdF absolute plate motion (APM). The mean mantle velocity is 7.85 ± 0.02 km/s with 4.6% ± 0.4% anisotropy. Synthesis of the plate-averaged Pn anisotropy signal with measurements of Pn anisotropy beneath the JdF Ridge and SKS splits across the JdF plate suggest that the anisotropic structure of the topmost mantle continues to evolve away from the spreading center to more closely align with APM. We infer that the oceanic mantle lithosphere may record the influence of both paleo-RPM and paleo-APM.PubDate: 2017-11-15T19:55:25.772448-05:DOI: 10.1002/2017GL074769

Authors:Paul D. Williams; M. Joan Alexander, Elizabeth A. Barnes, Amy H. Butler, Huw C. Davies, Chaim I. Garfinkel, Yochanan Kushnir, Todd P. Lane, Julie K. Lundquist, Olivia Martius, Ryan N. Maue, W. Richard Peltier, Kaoru Sato, Adam A. Scaife, Chidong ZhangAbstract: This paper synthesizes and summarizes atmospheric variability on time scales from seconds to decades through a phenomenological census. We focus mainly on unforced variability in the troposphere, stratosphere, and mesosphere. In addition to atmosphere-only modes, our scope also includes coupled modes, in which the atmosphere interacts with the other components of the Earth system, such as the ocean, hydrosphere, and cryosphere. The topics covered include turbulence on time scales of seconds and minutes, gravity waves on time scales of hours, weather systems on time scales of days, atmospheric blocking on time scales of weeks, the Madden–Julian Oscillation on time scales of months, the Quasi-Biennial Oscillation and El Niño–Southern Oscillation on time scales of years, and the North Atlantic, Arctic, Antarctic, Pacific Decadal, and Atlantic Multidecadal Oscillations on time scales of decades. The paper serves as an introduction to a special collection of Geophysical Research Letters on atmospheric variability. We hope that both this paper and the collection will serve as a useful resource for the atmospheric science community and will act as inspiration for setting future research directions.PubDate: 2017-11-15T10:00:03.35451-05:0DOI: 10.1002/2017GL075483

Authors:Y. Joh; E. Di LorenzoAbstract: The marine heatwave of 2014/2015 in the Northeast Pacific caused significant impacts on marine ecosystems and fisheries. While several studies suggest that land and marine heatwaves may intensify under climate change less is known about the prolonged multi-year nature (~2 years) of the Northeast Pacific events. Examination of reanalysis products and a 30-member climate model ensemble confirms that prolonged multi-year marine heatwaves are linked to the dynamics of the two dominant modes of winter sea surface temperature variability in the North Pacific, the Pacific Decadal Oscillation (PDO) and the North Pacific Gyre Oscillation (NPGO). Specifically, we find a significant correlation between winter warm NPGO anomalies and the following winter PDO arising from extra-tropical/tropical teleconnections. In the model projections for 2100 under the RCP8.5 scenario, this NPGO/PDO 1 year lag-correlation exhibits a significant positive trend (~35%) that favors more prolonged multi-year warm events (>1°C) with larger spatial coverage (~18%) and higher maximum amplitude (~0.5°C for events>2°C) over the Northeast Pacific.PubDate: 2017-11-13T17:37:00.211243-05:DOI: 10.1002/2017GL075930

Authors:Adel Imamovic; Linda Schlemmer, Christoph SchärAbstract: Ensembles of convection-resolving simulations with a simplified land surface are conducted to dissect the isolated and combined impacts of soil-moisture and orography on deep-convective precipitation under weak synoptic forcing. In particular, the deep-convective precipitation response to a uniform and a non-uniform soil-moisture perturbation is investigated both in settings with and without orography. In the case of horizontally uniform perturbations, we find a consistently positive soil moisture-precipitation feedback, irrespective of the presence of low orography. On the other hand, a negative feedback emerges with localized perturbations: a dry soil heterogeneity substantially enhances rain amounts that scale linearly with the dryness of the soil, while a moist heterogeneity suppresses rain amounts. If the heterogeneity is located in a mountainous region, the relative importance of soil-moisture heterogeneity decreases with increasing mountain height: A mountain 500m in height is sufficient to neutralize the local soil moisture-precipitation feedback.PubDate: 2017-11-13T17:36:54.224624-05:DOI: 10.1002/2017GL075657

Authors:Wen Zhou; Hanneke PaulssenAbstract: Noise interferometry has proven to be a powerful tool to image seismic structure. In this study we used data from 10 geophones located in a borehole at ∼3 km depth within the Groningen gas reservoir in the Netherlands. The continuous data cross-correlations show that noise predominantly comes in from above. The observed daily and weekly variations further indicate that the noise has an anthropogenic origin. The direct P wave emerges from the stacked vertical component cross-correlations with frequencies up to 80 Hz and the direct S wave is retrieved from the horizontal components with frequencies up to 50 Hz. The measured inter-geophone travel times were used to retrieve the P- and S-velocity structure along the borehole and a good agreement was found with well log data. In addition, from the S-wave polarizations, we determined azimuthal anisotropy with a fast direction of N65∘W±18∘ and an estimated magnitude of (4±2)%. The fast polarization direction corresponds to the present direction of maximum horizontal stress measured at nearby boreholes, but is also similar to the estimated paleostress direction.PubDate: 2017-11-13T17:36:51.701386-05:DOI: 10.1002/2017GL075592

Authors:Shin-Ya Ogino; Manabu D. Yamanaka, Shuichi Mori, Jun MatsumotoAbstract: We present a conceptual advance of the global water cycle in which the precipitation concentrated in tropical coastlines plays the role of an atmospheric dehydrator between the ocean and land. Landward water vapor transport peaks as it enters the coastal region (a few hundred kilometers offshore), and about half of the water vapor is consumed as precipitation over the coastal region before reaching the inland. Our results also revealed that the significant amount of net freshwater is supplied from the atmosphere to the coastal ocean, which is comparable to that of the land water discharge. This fact further provides a new insight on the ocean salinity distribution and its associated dynamics. We discuss a possible link between the tropical land distribution and the Earth's climate through the water circulation.PubDate: 2017-11-13T17:36:46.893671-05:DOI: 10.1002/2017GL075760

Authors:Meng-Hua Zhu; Kai Wünnemann, Natalia ArtemievaAbstract: We investigate how different temperature gradients of the Moon affect the ejection of lithic and molten material for impact basin several hundred kilometers in diameter to quantify the thickness and melt content of ejecta blanket as a function of radial distance. We find, by means of numerical modeling, that the ejecta thickness and melt content, similar to the basin formation, is sensitive to the thermal properties of the target. For two similar impact scenarios, the ejecta thickness with radial distance is proportional to a power-law, but for a “warm” target, it declines faster than for a “cold” target. In addition, the impact on the warm target produces more molten ejecta than in the case of the cold target. The thermal effects on the ejecta thickness distribution can be testified by the topographic variations around Imbrium and Orientale basins, which were thought to be formed on a warm and cold Moon, respectively. Our study demonstrates that the thermal effect needs to be taken into account to estimate the ejecta thickness distribution for large-scale impact basins on airless planetary surfaces.PubDate: 2017-11-13T17:36:45.117715-05:DOI: 10.1002/2017GL075405

Authors:Roberto E. Rizzo; David Healy, Natalie J. Farrell, Michael J. HeapAbstract: The mechanics of brittle failure is a well-described multi-scale process that involves a rapid transition from distributed microcracks to localisation along a single macroscopic rupture plane. However, considerable uncertainty exists regarding both the length scale at which this transition occurs and the underlying causes that prompt this shift from a distributed to a localised assemblage of cracks or fractures. For the first time, we used an image analysis tool developed to investigate orientation changes at different scales in images of fracture patterns in faulted materials, based on a two-dimensional continuous wavelet analysis. We detected the abrupt change in the fracture pattern from distributed tensile microcracks to localised shear failure in a fracture network produced by triaxial deformation of a sandstone core plug. The presented method will contribute to our ability of unravelling the physical processes at the base of catastrophic rock failure, including the nucleation of earthquakes, landslides and volcanic eruptions.PubDate: 2017-11-13T17:36:03.441552-05:DOI: 10.1002/2017GL075784

Authors:H. Afargan; Y. KaspiAbstract: This study investigates the occurrence of a midwinter suppression in synoptic eddy activity within the North Atlantic storm track. It is found that eddy kinetic energy over the Atlantic is reduced during winter relative to fall and spring, despite the stronger wintertime jet and enhanced baroclinicity. This behavior is similar to the well-known Pacific midwinter minimum, yet the reduction over the Atlantic is smaller and persists for a shorter period. To examine the conditions favorable for this phenomenon we present an analysis of years with stronger jet intensity versus years of weaker jets over the Atlantic and Pacific basins. When the wintertime jet is stronger, the midwinter suppression of eddy activity is more pronounced. Since the climatological Atlantic jet is weaker relative to the Pacific jet, the conditions for a midwinter suppression in the Atlantic are generally less favorable, yet a midwinter suppression often occurs in years of a strong jet.PubDate: 2017-11-13T17:35:46.254055-05:DOI: 10.1002/2017GL075136

Authors:Xiuli Yan; Min Nina Xu, Xianhui Sean Wan, Jin-Yu Terence Yang, Thomas W. Trull, Minhan Dai, Shuh-Ji KaoAbstract: Riverine nitrogen input into the coastal zone has increased remarkably in recent decades. Yet its transformation and recycling within hydrodynamically active regions remains unclear. Using observations of nitrate concentration and dual isotopic composition across the Changjiang River plume within a three end-member mixing model, we found deviations between the observed and expected values for mixing alone, revealing the non-conservative behavior of nitrate. Using cross correlations between concentrations and dual isotope deviations, we identified three nitrogen transformation zones, which correspond with separate portions of the plume. Nitrification and sedimentary denitrification occurred near the river mouth, nitrification prevailed further offshore under the plume, and finally phytoplankton assimilation in the outer surface plume (>100 km offshore), where it was fueled by nitrate that had already been strongly modified by microbial processes. Information was assembled into a conceptual model offering an overview of nitrogen transformations in a large river plume.PubDate: 2017-11-13T17:35:44.336893-05:DOI: 10.1002/2017GL075951

Authors:Brian F. Thomas; Júlio Caineta, Jamiat NantezaAbstract: The world's largest aquifers are a fundamental source of freshwater used for agricultural irrigation and to meet human water needs. Therefore, their stored volume of groundwater is linked with water security, which becomes more relevant during periods of drought. This work focuses on understanding large-scale groundwater changes, where we introduce an approach to evaluate groundwater sustainability at a global scale. We employ a groundwater drought index to assess performance metrics (reliability, resilience, vulnerability and a combined sustainability index) for the largest and most productive global aquifers. Spatiotemporal changes in total water storage are derived from remote sensing observations of gravity anomalies, from which the groundwater drought index is inferred. The results reveal a complex relationship between the indicators, while considering monthly variability in groundwater storage. Combining the drought and sustainability indexes, as presented in this work, constitutes a measure for quantifying groundwater sustainability. This framework integrates changes in groundwater resources due to human influences and climate changes, thus opening a path to assess progress towards sustainable use and water security.PubDate: 2017-11-13T17:35:41.261934-05:DOI: 10.1002/2017GL076005

Authors:Zhixue Du; Colin Jackson, Neil Bennett, Peter Driscoll, Jie Deng, Kanani K. M. Lee, Eran Greenberg, Vitali B. Prakapenka, Yingwei FeiAbstract: The origin of Earth's ancient magnetic field is an outstanding problem. It has recently been proposed that exsolution of MgO from the core may provide sufficient energy to drive an early geodynamo. Here we present new experiments on Mg partitioning between iron-rich liquid and silicate/oxide melt. Our results indicate that Mg partitioning depends strongly on the oxygen content in iron-rich liquid, in contrast to previous finding that it depends only on temperature. Consequently, MgO exsolution during core cooling is drastically reduced and insufficient to drive an early geodynamo alone. Using the new experimental data, our thermal model predicts inner core nucleation at ~850 Ma and a nearly constant paleointensity.PubDate: 2017-11-13T17:35:38.32619-05:0DOI: 10.1002/2017GL075283

Authors:A. K. Patra; P. Pavan Chaitanya, J. P. St.- Maurice, Y. Otsuka, T. Yokoyama, M. YamamotoAbstract: Radar echoes from the daytime equatorial ionospheric F1 region, popularly known as ‘150-km echoes’, have challenged ionospheric plasma physicists for several decades. Recent theoretical simulations showed that enhanced photo-electron fluxes can amplify the amplitude of plasma waves, generating spectra similar to those of the radar echoes, implying that larger solar fluxes should produce more frequent and stronger 150-km echoes. Inspired by this proposal we studied the occurrence and intensity dependence of the echoes on the EUV flux observed by SOHO over several years. The occurrence and intensity of the echoes were found to have an inverse relationship with this EUV flux measurement. The multi-year trend is independent of the variability often observed over successive days with nearly identical EUV fluxes. These results imply that the relationship between the echoes and EUV flux is more complex. We propose that gravity waves modulate the amplitude of 150-km echoes through changes in the variations in plasma density and photoelectron fluxes associated with the gravity wave induced neutral density modulations.PubDate: 2017-11-13T17:35:31.772541-05:DOI: 10.1002/2017GL074678

Authors:Xiaowen Zhang; Thomas S. Bianchi, Xingqian Cui, Brad E. Rosenheim, Chien-Lu Ping, Andrea J. M. Hanna, Mikhail Kanevskiy, Kathryn M. Schreiner, Mead A. AllisonAbstract: The deposition of terrestrial-derived permafrost particulate organic carbon (POC) has been recorded in major Arctic river deltas. However, associated transport pathways of permafrost POC from the watershed to the coast have not been well-constrained. Here, we utilized a combination of ramped pyrolysis-oxidation radiocarbon analysis (RPO 14C) along with lignin biomarkers, to track the linkages between soils and river and delta sediments. Surface and deep soils showed distinct RPO thermographs which may be related to degradation and organo-mineral interaction. Soil material in the bedload of the river channel was mostly derived from deep old permafrost. Both surface and deep soils were transported and deposited to the coast. Hydrodynamic sorting and barrier island protection played important roles in terrestrially-derived permafrost POC deposition near the coast. On a large scale, ice processes (e.g., ice gauging and strudel scour) and ocean currents controlled the transport and distribution of permafrost POC on Beaufort Shelf.PubDate: 2017-11-13T17:35:28.777269-05:DOI: 10.1002/2017GL075543

Authors:Hamed Moftakhari; Amir AghaKouchak, Brett F. Sanders, Richard A. Matthew, Omid MazdiyasniAbstract: Climate change may affect ocean-driven coastal flooding regimes by both raising the mean sea level (MSL) and altering ocean-atmosphere interactions. For reliable projections of coastal flood risk, information provided by different climate models must be considered in addition to associated uncertainties. In this paper, we propose a framework to project future coastal water levels and quantify the resulting flooding hazard to infrastructure. We use Bayesian Model Averaging to generate a weighted ensemble of storm surge predictions from eight climate models for two coastal counties in California. The resulting ensembles combined with MSL projections and predicted astronomical tides are then used to quantify changes in the likelihood of road flooding under representative concentration pathways 4.5 and 8.5 in near-future (1998-2063) and mid-future (2018-2083). The results show that, road flooding rates will be significantly higher in near-future and mid-future compared to the recent past (1950-2015) if adaptation measures are not implemented.PubDate: 2017-11-13T17:35:25.705757-05:DOI: 10.1002/2017GL076116

Authors:Hanin Binder; Maxi Boettcher, Christian M. Grams, Hanna Joos, Stephan Pfahl, Heini WernliAbstract: At the turn of the years 2015/2016, maximum surface temperature in the Arctic reached record-high values, exceeding the melting point, which led to a strong reduction of the Arctic sea-ice extent in the middle of the cold season. Here we show, using a Lagrangian method, that a combination of very different airstreams contributed to this event: (i) warm low-level air of subtropical origin, (ii) initially cold low-level air of polar origin heated by surface fluxes, and (iii) strongly descending air heated by adiabatic compression. The poleward transport of these warm airstreams occurred along an intense low-level jet between a series of cyclones and a quasi-stationary anticyclone. The complex 3D configuration that enabled this transport was facilitated by continuous warm conveyor belt ascent into the upper part of the anticyclone. This study emphasises the combined role of multiple transport processes and transient synoptic-scale dynamics for establishing an extreme Arctic warm event.PubDate: 2017-11-10T18:01:30.52043-05:0DOI: 10.1002/2017GL075841

Authors:Q. Coopman; T. J. Garrett, D. P. Finch, J. RiediAbstract: The rate of warming in the Arctic depends upon the response of low-level microphysical and radiative cloud properties to aerosols advected from distant anthropogenic and biomass-burning sources. Cloud droplet cross-section density increases with higher concentrations of cloud condensation nuclei, leading to an increase of cloud droplet absorption and scattering radiative cross-sections. The challenge of assessing the magnitude of the effect has been decoupling the aerosol impacts on clouds from how clouds change solely due to natural meteorological variability. Here we address this issue with large, multi-year satellite, meteorological, and tracer transport model datasets to show that the response of low-level clouds in the Arctic to anthropogenic aerosols lies close to a theoretical maximum and is between two and eight times higher than has been observed elsewhere. However, a previously described response of arctic clouds to biomass-burning plumes appears to be overstated because the interactions are rare and modification of cloud radiative properties appears better explained by coincident changes in temperature, humidity, and atmospheric stability.PubDate: 2017-11-09T17:20:54.871779-05:DOI: 10.1002/2017GL075795

Authors:Yetang Wang; Elizabeth R. Thomas, Shugui Hou, Baojuan Huai, Shuangye Wu, Weijun Sun, Shanzhong Qi, Minghu Ding, Yulun ZhangAbstract: This study uses a set of 37 firn core records over the West Antarctic Ice Sheet (WAIS) to test the performance of ERA-20C reanalysis for snow accumulation and quantify temporal variability in snow accumulation since 1900. The firn cores are allocated to four geographical areas demarcated by drainage divides (i.e., Antarctic Peninsula (AP), western WAIS, central WAIS and eastern WAIS) to calculate stacked records of regional snow accumulation. Our results show that the inter-annual variability in ERA-20C precipitation minus evaporation (P-E) agrees well with the corresponding ice core snow accumulation composites in each of the four geographical regions, suggesting its skill for simulating snow accumulation changes before the modern satellite era (pre-1979). Snow accumulation experiences significantly positive trends for the AP and eastern WAIS, a negative trend for the western WAIS, and no significant trend for the central WAIS from 1900 to 2010. The contrasting trends are associated with changes in the large-scale moisture transport driven by a deepening of the low-pressure systems and anomalies of sea ice in the Amundsen Sea Low (ASL) region.PubDate: 2017-11-09T17:20:35.34677-05:0DOI: 10.1002/2017GL075135

Authors:S. Kameda; S. Ikezawa, M. Sato, M. Kuwabara, N. Osada, G. Murakami, K. Yoshioka, I. Yoshikawa, M. Taguchi, R. Funase, S. Sugita, Y. Miyoshi, M. FujimotoAbstract: The hydrogen exosphere constitutes the uppermost atmospheric layer of the Earth, and its shape may reflect the last stage of the atmospheric escape process. The distribution of hydrogen in the outer exosphere remains unobserved because outer geocoronal emissions are difficult to observe from within the exosphere. In this study, we used the Lyman Alpha Imaging Camera (LAICA) onboard the Proximate Object Close Flyby with Optical Navigation (PROCYON) spacecraft, located outside the exosphere, to obtain the first image of the entire geocorona that extends to more than 38 Earth radii. The observed emission intensity distribution can be reproduced using our analytical model that has three parameters: exobase temperature, exobase density, and solar radiation pressure, which implies that hot hydrogen production in the magnetized plasmasphere is not the dominant process shaping the outer hydrogen exosphere. However, the role of the magnetic effect in determining the total escape flux cannot be ruled outPubDate: 2017-11-09T17:20:24.797648-05:DOI: 10.1002/2017GL075915

Authors:Sarah A. Safieddine; Colette L. HealdAbstract: Precipitation is the largest physical removal pathway of atmospheric reactive organic carbon in the form of dissolved organic carbon (DOC). We present the first global DOC distribution simulated with a global model. A total of 85 and 188 TgCyr-1 are deposited to the ocean and the land respectively, with DOC ranging between 0.1 and 10 mgCL-1 in this GEOS-Chem simulation. We compare the 2010 simulated DOC to a 30-year synthesis of measurements. Despite limited measurements and imperfect temporal matching, the model is able to reproduce much of the spatial variability of DOC (r= 0.63), with a low bias of 35%. We present the global average carbon oxidation state (OSc¯) as a simple metric for describing the chemical composition. In the atmosphere, −1.8≤OSc¯≤−0.6 and the increase in solubility upon oxidation leads to a global increase in OSc¯ in precipitation with −0.6≤OSc¯DOC≤0.PubDate: 2017-11-08T17:49:14.2694-05:00DOI: 10.1002/2017GL075270

Authors:S. K. Kharol; M. W. Shephard, C. A. McLinden, L. Zhang, C. E. Sioris, J. M. O’Brien, R. Vet, K. E. Cady-Pereira, E. Hare, J. Siemons, N. A. KrotkovAbstract: Reactive nitrogen (Nr) is an essential nutrient to plants and a limiting element for growth in many ecosystems, but it can have harmful effects on ecosystems when in excess. Satellite-derived surface observations are used together with a dry deposition model to estimate the dry deposition flux of the most abundant short-lived nitrogen species, NH3 and NO2, over North America during the 2013 warm season. These fluxes demonstrate that the NH3 contribution dominates over NO2 for most regions (comprising ~85% of their sum in Canada, and ~65% in US), with some regional exceptions (e.g. Alberta and northeastern-US). Nationwide, ~51 tonnes(N) of these species were dry deposited in the US, approximately double the ~28 tonnes(N) in Canada over this period. Forest fires are shown to be the major contributor of dry deposition of Nr from NH3 in northern latitudes, and tend to lead to deposition fluxes 2-3 times greater than from expected amounts without fires.PubDate: 2017-11-08T17:49:11.199716-05:DOI: 10.1002/2017GL075832

Authors:P. Corlies; A. G. Hayes, S. P. D. Birch, R. Lorenz, B. W. Stiles, R. Kirk, V. Poggiali, H. Zebker, L. IessAbstract: With the conclusion of the Cassini mission, we present an updated topographic map of Titan, including all the available altimetry, SARtopo, and stereo-photogrammetry topographic datasets available from the mission. We use radial basis functions to interpolate the sparse dataset, which covers only ∼9% of Titan's global area. The most notable updates to the topography include higher coverage of the poles of Titan, improved fits to the global shape, and a finer resolution of the global interpolation. We also present a statistical analysis of the error in the derived products and perform a global minimization on a profile-by-profile basis to account for observed biases in the input dataset. We find a greater flattening of Titan than measured, additional topographic rises in Titan's southern hemisphere, and better constrain the possible locations of past and present liquids on Titan's surface.PubDate: 2017-11-07T19:11:09.812324-05:DOI: 10.1002/2017GL075518

Authors:Liming He; Jing M. Chen, Holly Croft, Alemu Gonsamo, Xiangzhong Luo, Jane Liu, Ting Zheng, Ronggao Liu, Yang LiuAbstract: The magnitude and variability of the terrestrial CO2 sink remain uncertain, partly due to limited global information on ecosystem nitrogen (N) and its cycle. Without N constraint in ecosystem models, the simulated benefits from CO2 fertilization and CO2-induced increases in water use efficiency (WUE) may be overestimated. In this study, satellite observations of a relative measure of chlorophyll content are used as a proxy for leaf photosynthetic N content globally for 2003–2011. Global gross primary productivity (GPP) and evapotranspiration are estimated under elevated CO2 and N-constrained model scenarios. Results suggest that the rate of global GPP increase is overestimated by 85% during 2000-2015 without N limitation. This limitation is found to occur in many tropical and boreal forests, where a negative leaf N trend indicates a reduction in photosynthetic capacity, thereby suppressing the positive vegetation response to enhanced CO2 fertilization. Based on our carbon-water coupled simulations, enhanced CO2 concentration decreased stomatal conductance and hence increased WUE by 10% globally over the 1982 to 2015 time frame. Due to increased anthropogenic N application, GPP in croplands continues to grow and offset the weak negative trend in forests due to N limitation. Our results also show that the improved WUE is unlikely to ease regional droughts in croplands because of increases in evapotranspiration, which are associated with the enhanced GPP. Although the N limitation on GPP increase is large, its associated confidence interval is still wide, suggesting an urgent need for better understanding and quantification of N limitation from satellite observations.PubDate: 2017-11-06T18:26:33.78642-05:0DOI: 10.1002/2017GL075981

Authors:Yasmina M. Martos; Manuel Catalan, Tom A. Jordan, Alexander Golynsky, Dmitry Golynsky, Graeme Eagles, David G. VaughanAbstract: Antarctica is the largest reservoir of ice on Earth. Understanding its ice sheet dynamics is crucial to unraveling past global climate change and making robust climatic and sea level predictions. Of the basic parameters that shape and control ice flow, the most poorly known is geothermal heat flux. Direct observations of heat flux are difficult to obtain in Antarctica, and until now continent-wide heat flux maps have only been derived from low-resolution satellite magnetic and seismological data. We present a high resolution heat flux map and associated uncertainty derived from spectral analysis of the most advanced continental compilation of airborne magnetic data. Small-scale spatial variability and features consistent with known geology are better reproduced than in previous models, between 36% and 50%. Our high-resolution heat-flux map and its uncertainty distribution provide an important new boundary condition to be used in studies on future subglacial hydrology, ice-sheet dynamics and sea-level change.PubDate: 2017-11-06T18:26:29.181461-05:DOI: 10.1002/2017GL075609

Authors:L. B. Wheeler; J. GalewskyAbstract: The development of relief may generate leeside rain shadows where precipitation δ18O values are lower due to rainout on the windward side. The magnitude of lowering in paleo-δ18O sampled from the leeside of a mountain range should, at least in principle, be related to the elevation of the mountain range. In order for leeside proxies to record the highest elevations, the majority of air masses need to travel up and over the ridge crest before raining out on the leeside. If atmospheric flow patterns around the mountain range are dominated by flow deflection, air masses that reach the leeside are less likely to record the highest elevations. Using the Weather Research and Forecasting model and Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model, we demonstrate that modern atmospheric flow patterns in the Southern Alps of New Zealand are not dominated by flow deflection. The lack of flow deflection around the Southern Alps and the relatively low relief throughout the uplift history supports the use of leeside isotope records to constrain the timing of uplift, and that uplift likely occurred ~5 Ma based on leeside isotope records. In contrast, orogens that are characterized by high elevations and strong flow deflection, like the Sierra Nevada of California, may not be good candidates for leeside isotope-based paleoaltimetry studies. Ideal candidates for such studies should be characterized by relatively low elevations, low atmospheric flow deflection, and relatively low relief throughout the period of uplift.PubDate: 2017-11-06T18:26:23.341812-05:DOI: 10.1002/2017GL074753

Authors:L. Y. Li; B. Liu, J. Yu, J. B. CaoAbstract: Under different solar wind pressures, we observed the magnetosonic (MS) wave amplification and attenuation associated with the compression and expansion of the Earth's magnetosphere. By analyzing the wave and particle variations recorded by the twin Van Allen Probes, we found that the magnetospheric compression or expansion can alter the keV proton phase space density distribution in velocity space and thus affects the MS wave intensity in upper band (f> 50Hz) in the dawnside magnetosphere (MLT ~ 4.0 – 7.9 and L ~ 5.7 – 3.0). During the magnetospheric compression period, the reduction of the 0.1 to 2keV protons and the enhancement of the 3 to 7keV protons form a positive phase space density gradient in their velocity space (i.e., the proton ring distribution), and meanwhile the upper band MS waves are significantly amplified in the proton ring distribution region. During the subsequent magnetospheric expansion period, the enhancement of the 0.1 to 2keV protons and the reduction of the protons above 3keV produce a negative phase space density gradient in their velocity space, and meanwhile the upper band MS waves are rapidly damped. These observations demonstrate that the intensity of the upper band MS waves in the dawnside magnetosphere is highly variable during the change in solar wind pressure.PubDate: 2017-11-06T18:26:22.055432-05:DOI: 10.1002/2017GL075649

Authors:Yuan Wang; Martin Claus, Richard J. Greatbatch, Jinyu ShengAbstract: We show how a barotropic shallow water model can be used to decompose the mean barotropic transport from a high-resolution ocean model based on the vertically-averaged momentum equations. We apply the method to a high resolution model of the North Atlantic for which the local vorticity budget is both noisy and dominated by small spatial scales. The shallow water model acts as an effective filter and clearly reveals the transport driven by each term. The potential energy (JEBAR) term is the most important for driving transport, including in the northwest corner, while mean flow advection is important for driving transport along f/H contours around the Labrador Sea continental slope. Both the eddy momentum flux and the mean flow advection terms drive significant transport along the pathway of the Gulf Stream and the North Atlantic Current.PubDate: 2017-11-06T18:26:19.455116-05:DOI: 10.1002/2017GL074825

Authors:Marianne Sloth Madsen; Peter L. Langen, Fredrik Boberg, Jens Hesselbjerg ChristensenAbstract: Multi-model ensembles are widely analyzed to estimate the range of future regional climate change projections. For an ensemble of climate models, the result is often portrayed by showing maps of the geographical distribution of the multi-model mean results and associated uncertainties represented by model spread at the grid point scale. Here we use a set of CMIP5 models to show that presenting statistics this way results in an overestimation of the projected range leading to physically implausible patterns of change on global but also on regional scales. We point out that similar inconsistencies occur in impact analyses relying on multi-model information extracted using statistics at the regional scale, e.g. when a sub-set of CMIP models is selected to represent regional model spread. Consequently, the risk of unwanted impacts may be overestimated at larger scales as climate change impacts will never be realized as the worst (or best) case everywhere.PubDate: 2017-11-06T18:26:17.218121-05:DOI: 10.1002/2017GL075627

Authors:Thiago P. Santos; Douglas O. Lessa, Igor M. Venancio, Cristiano M. Chiessi, Stefan Mulitza, Henning Kuhnert, Ana Luiza S. AlbuquerqueAbstract: After glacial terminations, large amounts of heat and salt were transferred from low- to high-latitudes, which is a crucial phenomenon for the reestablishment of the Atlantic Meridional Overturning Circulation (AMOC). However, how different glacial terminations evolved in the (sub)tropics is still poorly documented. Here, we use foraminifera oxygen (δ18O) and carbon (δ13C) stable isotopes to show that the North Atlantic heat piracy, following the AMOC resumption at the early Last Interglacial, affected the thermocline δ18O levels of the subtropical western South Atlantic. Because of the cooling imposed by this process, glacial δ18O persisted in the thermocline for ~ 7 kyr after the onset of the Last Interglacial, dampening the effect of sea-level rise usually imprinted on foraminifera δ18O during terminations. Faunal composition and δ13C also suggest the existence of a colder and thicker South Atlantic Central Water coeval with the AMOC recovery. This process apparently did not occur during the last deglaciation.PubDate: 2017-11-06T18:26:06.514415-05:DOI: 10.1002/2017GL074457

Authors:J. B. SnivelyAbstract: Numerical simulations demonstrate theoretical predictions that gravity waves with short periods (∼4-8 minutes) in the mesosphere and lower thermosphere may force secondary acoustic waves, with harmonic periods (∼2-4 minutes), that can reach detectable amplitudes in the thermosphere and ionosphere. The mechanism is through their vertical fluxes of vertical momentum, which lead to forcing as they are disrupted by varying stratification or instability. This is shown likely to occur where horizontally- or radially-opposing gravity waves interact at large amplitudes, such as above large convective sources, and after overturning. Evanescence and reflection of the waves can lead to further enhancements of the vertical fluxes and the potential for forcing. Results thus identify one of likely several mechanisms for the nonlinear conversion from gravity waves to acoustic waves, to elucidate an unappreciated source of vertical coupling.PubDate: 2017-11-06T18:26:04.411265-05:DOI: 10.1002/2017GL075360

Authors:Gareth J. Marshall; David W. J. Thompson, Michiel R. BroekeAbstract: We provide the first comprehensive analysis of the relationships between large-scale patterns of Southern Hemisphere climate variability and the detailed structure of Antarctic precipitation. We examine linkages between the high spatial resolution precipitation from a regional atmospheric model and four modes of large-scale Southern Hemisphere climate variability: the southern baroclinic annular mode (BAM), the southern annular mode (SAM), and the two Pacific-South American (PSA) teleconnection patterns. Variations in all four modes influence the spatial patterns of precipitation over Antarctica, consistent with their signatures in high-latitude meridional moisture fluxes. They impact not only the mean but also the incidence of extreme precipitation events. Current coupled-climate models are able to reproduce all four patterns of atmospheric variability, but struggle to correctly replicate their regional impacts on Antarctic climate. Thus, linking these patterns directly to Antarctic precipitation variability may allow a better estimate of future changes in precipitation than using model output alone.PubDate: 2017-11-06T18:25:59.605799-05:DOI: 10.1002/2017GL075998

Authors:Cecilia Peralta-Ferriz; Rebecca A. WoodgateAbstract: It is typically stated that the Pacific-to-Arctic oceanic flow through the Bering Strait (important for Arctic heat, freshwater, and nutrient budgets) is driven by local wind and a (poorly defined) far-field “pressure-head” forcing, related to sea-surface-height differences between the Pacific and the Arctic. Using monthly, Arctic-wide, ocean bottom pressure satellite data and in situ mooring data from the Bering Strait from 2002-2016, we discover the spatial structure of this pressure-head forcing, finding that the Bering Strait throughflow variability is dominantly driven from the Arctic, specifically by sea-level change in the East Siberian Sea (ESS), in turn related to westward winds along the Arctic coasts. In the (comparatively calm) summer, this explains ~2/3rds of the Bering Strait variability. In winter, local wind variability dominates the total flow, but the pressure-head term, while still correlated with the ESS-dominated sea-level pattern, is now more strongly related to Bering Sea shelf sea-level variability.PubDate: 2017-11-06T18:25:55.969585-05:DOI: 10.1002/2017GL075179

Authors:Pedro N. DiNezio; Clara Deser, Alicia Karspeck, Stephen Yeager, Yuko Okumura, Gokhan Danabasoglu, Nan Rosenbloom, Julie Caron, Gerald A. MeehlAbstract: Historical observations show that one in two La Niña events last for two consecutive years. Despite their outsized impacts on drought, these 2-year La Niña are not predicted on a routine basis. Here, we assess the predictability of 2-year La Niña using retrospective forecasts performed with a climate model that simulates realistic multi-year events, as well as with an empirical model based on observed predictors. The skill of the retrospective forecasts allows us to make predictions for the upcoming 2017-2018 boreal winter starting from conditions in November 2015. These two-year forecasts indicate that the return of La Niña is more likely than not, with a 60% probability based on the climate model and an 80% probability based on the empirical model; the likelihood of El Niño is less than 8% in both cases. These results demonstrate the feasibility of predictions of the duration of La Niña.PubDate: 2017-11-06T18:25:54.290702-05:DOI: 10.1002/2017GL074904

Authors:Yi Wang; Ingrid Hendy, Tiffany J. NapierAbstract: Understanding dissolved oxygen variability in the ocean is limited by the short duration of direct measurements, however sedimentary oxidation-reduction reactions can provide context for modern observations. Here we use bulk sediment redox-sensitive metal enrichment factors (MoEF, ReEF, and UEF) and scanning X-ray fluorescence (XRF) records to examine annual-scale sedimentary oxygen concentrations in the Santa Barbara Basin from the Industrial Revolution (AD ~1850) to present. Enrichments are linked to measured bottom water oxygen concentrations after 1986. We reveal gradual intensification of the coastal oxygen minimum zone (OMZ) on the southern California margin coinciding the 20th century anthropogenic warming trend that leads to reduced oxygen solubility and greater stratification. High-frequency interannual oscillations become more prominent over the last three decades. These are attributed to local ‘flushing events’ triggered by the transition from El Niño to La Niña conditions, which further amplify changes in the extratropical southern Californian OMZ.PubDate: 2017-11-06T18:25:44.220654-05:DOI: 10.1002/2017GL075443

Authors:Shilpa Gahlot; Shijie Shu, Atul K. Jain, Somnath Baidya RoyAbstract: In this paper we explore the trend in Net Biome Productivity (NBP) over India for the period 1980-2012 and quantify the impact of different environmental factors, including atmospheric CO2 concentrations ([CO2]), land use and land cover change (LULCC), climate and nitrogen deposition on carbon fluxes using a land surface model, Integrated Science Assessment Model (ISAM). Results show that terrestrial ecosystems of India have been a carbon sink for this period. Driven by a strong CO2 fertilization effect, magnitude of NBP increased from 27.17 TgC/yr in the 1980s to 34.39 TgC/yr in the 1990s, but decreased to 23.70 TgC/yr in the 2000s due to change in climate. Adoption of forest conservation, management and reforestation policies in the past decade has promoted carbon sequestration in the ecosystems but this effect has been offset by loss of carbon from ecosystems due to rising temperatures and decrease in precipitation.PubDate: 2017-11-06T18:25:33.386741-05:DOI: 10.1002/2017GL075777

Authors:K. P. Jochum; J. A. Schuessler, X.-H. Wang, B. Stoll, U. Weis, W. E. G. Müller, G. H. Haug, M. O. Andreae, P. N. FroelichAbstract: Silicon is a keystone nutrient in the ocean for understanding climate change because of the importance of Southern Ocean diatoms in taking up CO2 from the surface ocean–atmosphere system and sequestering carbon into the deep sea. Here we report on silicon isotopes and germanium-to-silicon ratios in giant glass spicules of deep-sea sponge Monorhaphis chuni over the past 17,000 years. In-situ measurements of Si isotopes and Ge concentrations show systematic variations from rim to center of the cross sections. When calibrated against seawater concentrations using data from modern spicule rims, sponge data indicate that dissolved silica concentrations in the deep Pacific were ~12 % higher during the early deglacial. These deep Pacific Ocean data help to fill an important global gap in paleo-nutrient records. Either continental sources supplied more silica to the deglacial ocean and/or biogenic silica burial was lower, both of which may have affected atmospheric CO2.PubDate: 2017-11-06T18:25:29.958354-05:DOI: 10.1002/2017GL073897

Authors:Michelle L. L'Heureux; Michael K. Tippett, Arun Kumar, Amy H. Butler, Laura M. Ciasto, Qinghua Ding, Kirstin J. Harnos, Nathaniel C. JohnsonAbstract: Arctic Oscillation (AO) variability impacts climate anomalies over the mid-to-high latitudes of the Northern Hemisphere. Recently, state-of-the-art climate prediction models have proved capable of skillfully predicting the AO during the winter, revealing a previously unrealized source of climate predictability. Hindcasts from the North American Multi-Model Ensemble (NMME) show that the seasonal, ensemble mean 200-hPa AO index is skillfully predicted up to seven months in advance and that this skill, especially at longer leads, is coincident with previously unknown and strong relations (r> 0.9) with the El Niño-Southern Oscillation (ENSO). The NMME is a seasonal prediction system that comprises eight models and up to 100 members with forecasts out to 12 months. Observed ENSO-AO correlations are within the spread of the NMME member correlations, but the majority of member correlations are stronger than observed, consistent with too high predictability in the model, or over-confidence.PubDate: 2017-11-06T18:25:24.942704-05:DOI: 10.1002/2017GL074854

Authors:Christophe Zaroli; Paula Koelemeijer, Sophie LambotteAbstract: Geophysical tomographic studies traditionally exploit linear, damped least-squares inversion methods. We demonstrate that the resulting models can be locally biased toward lower or higher amplitudes in regions of poor data illumination, potentially causing physical misinterpretations. For example, we show that global model S40RTS is locally biased toward higher amplitudes below isolated receivers where ray paths are quasi vertical, such as on Hawaii. This leads to questions on the apparent low-velocity structure interpreted as the Hawaii hotspot. We prove that a linear Backus–Gilbert inversion scheme can bring the Earth's interior into focus through unbiased tomographic lenses, as its model estimates are constrained to be averages over the true model. It also efficiently computes the full generalized inverse required to infer both model resolution and its covariance, enabling quantitative interpretations of tomographic models.PubDate: 2017-11-02T17:01:23.763355-05:DOI: 10.1002/2017GL074996

Authors:Thomas S. Coffey; John B. ShawAbstract: We show that distributary channels on river deltas exhibit a mean bifurcation angle that can be understood using theory developed in tributary channel networks. In certain cases, tributary network bifurcation geometries have been demonstrated to be controlled by diffusive groundwater flow feeding incipient bifurcations, producing a characteristic angle of 72∘. We measured 25 unique distributary bifurcations in an experimental delta and 197 bifurcations in 10 natural deltas, yielding a mean angle of 70.4∘±2.6∘(95% confidence interval) for field-scale deltas and a mean angle of 68.3∘±8.7∘ for the experimental delta, consistent with this theoretical prediction. The bifurcation angle holds for small scales relative to channel width length-scales. Furthermore, the experimental data show that the mean angle is 72∘ immediately after bifurcation initiation and remains relatively constant over significant timescales. Although distributary networks do not mirror tributary networks perfectly, the similar control and expression of bifurcation angles suggests that additional morphodynamic insight may be gained from further comparative study.PubDate: 2017-11-02T17:01:13.4761-05:00DOI: 10.1002/2017GL074873

Authors:E. A. Cougnon; B. K. Galton-Fenzi, S. R. Rintoul, B. Legrésy, G. D. Williams, A. D. Fraser, J. R. HunterAbstract: Ice shelf basal melt is the dominant contribution to mass loss from Antarctic ice shelves. However, the sensitivity of basal melt to changes in icescape (grounded icebergs, ice shelves and sea ice) and related ocean circulation is poorly understood. Here, we simulate the impact of the major 2010 calving event of the Mertz Glacier Tongue (MGT), East Antarctica, and related redistribution of sea ice and icebergs on the basal melt rate of the local ice shelves. We find that the position of the grounded tabular iceberg B9B controls the water masses that reach the nearby ice shelf cavities. After the calving of the MGT and the removal of B9B, warmer water is present both within the MGT cavity and on the continental shelf driving a 57% increase of the deep MGT basal melting. Major changes in icescape influence the oceanic heat flux responsible for basal ice shelf melting.PubDate: 2017-11-02T17:00:59.21638-05:0DOI: 10.1002/2017GL074943

Authors:Chae Kyung Sim; Sungsoo S. Kim, Paul G. Lucey, Ian Garrick-Bethell, Young-Jun ChoiAbstract: Using new topography-corrected spectral data from the SELENE spacecraft, here we report a new lunar crater property produced by space weathering. We find the optical properties of north, south, east, and west walls vary systematically across the Moon; pole-facing walls are brighter and less red (i.e., less mature) than their equator-facing counterparts as latitude increases, which we explain by reduced solar wind flux in pole-facing slopes. On the nearside, we find that east-west differences in crater wall brightness and redness vary with longitude, which we explain by solar wind shielding as the Moon passes through the Earth's magnetosphere. Because micrometeoroids are largely unaffected by magnetosphere passage, the longitudinal effect is used to discriminate between micrometeoroid and solar wind effects. Thus, for the first time we quantify how surface optical properties vary with solar wind flux.PubDate: 2017-11-02T17:00:48.819339-05:DOI: 10.1002/2017GL075338

Authors:M. Mouyen; A. Canitano, B. F. Chao, Y.-J. Hsu, P. Steer, L. Longuevergne, J.-P. BoyAbstract: Geodetic instruments now offer compelling sensitivity, allowing to investigate how solid Earth and surface processes interact. By combining surface air pressure data, non-tidal sea level variations model and rainfall data, we systematically analyze the volumetric deformation of the shallow crust at 7 borehole strainmeters in Taiwan induced by 31 tropical cyclones (typhoons) that made landfall to the island from 2004 to 2013. The typhoon's signature consists in a ground dilatation due to air pressure drop, generally followed by a larger ground compression. We show that this compression phase can be mostly explained by the mass loading of rainwater that falls on the ground and concentrates in the valleys towards the strainmeter sensitivity zone. Further, our analysis shows that borehole strainmeters can help quantifying the amount of rainwater accumulating and flowing over a watershed during heavy rainfalls, which is a useful constraint for building hydrological models.PubDate: 2017-11-02T17:00:46.119448-05:DOI: 10.1002/2017GL075615

Authors:Saïd Qasmi; Christophe Cassou, Julien BoéAbstract: A realistic simulation of the Atlantic Multidecadal Variability (AMV) and related teleconnections is essential to resolve and understand the potential predictability over Europe at decadal time-scale. Based on a large ensemble of state-of-the-art climate models, we show that a considerable inter-model spread exists in the spatio-temporal properties of the simulated AMV and teleconnections with European summer temperature. The greater the persistence, variance and basin-scale spatial coherence, the stronger the teleconnection. We demonstrate that only a few members of a few models produce a teleconnection that is consistent with observational estimates over the instrumental period. This highlights the possible extreme nature of the last century teleconnection and/or a detrimental underestimation of ocean-land teleconnectivity in many climate models. Yet, we emphasize the considerable uncertainties due to methods used to disentangle internal and externally forced variations in observations, and to sampling, which must be correctly accounted when analyses are performed on short temporal records.PubDate: 2017-11-02T17:00:40.695724-05:DOI: 10.1002/2017GL074886

Authors:Shaosui Xu; David Mitchell, Janet Luhmann, Yingjuan Ma, Xiaohua Fang, Yuki Harada, Takuya Hara, David Brain, Tristan Webber, Christian Mazelle, Gina A. DiBraccioAbstract: With electron and magnetic field data obtained by the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft, we have identified closed magnetic field lines, with both footpoints embedded in the dayside ionosphere, extending up to 6200 km altitude into the Martian tail. This topology is deduced from photoelectrons produced in the dayside ionosphere being observed traveling both parallel and anti-parallel to the magnetic field. At trapped-zone pitch angles (within a range centered on 90° where electrons magnetically reflect before interacting with the atmosphere), cases with either solar wind electrons or photoelectrons have been found, indicating different formation mechanisms for these closed loops. These large closed loops are present in MHD simulations. The case with field-aligned photoelectrons mixed with solar wind electrons having trapped-zone pitch angles is likely to be associated with reconnection, while the case with photoelectrons at all pitch angles is probably due to closed field lines being pulled tailward by the surrounding plasma flow. By utilizing an algorithm for distinguishing photoelectrons from solar wind electrons in pitch angle resolved energy spectra, we systematically map the spatial distribution and occurrence rate of these closed magnetic loops over the region sampled by the MAVEN orbit. We find that the occurrence rate ranges from a few percent to a few tens of percent outside of the optical shadow and less than one percent within the shadow. These observations can be used to investigate the general magnetic topology in the tail, which is relevant to cold ion escape, reconnection, and flux ropes.PubDate: 2017-11-02T17:00:34.668089-05:DOI: 10.1002/2017GL075831

Authors:Michael Aldam; Marc Weikamp, Robert Spatschek, Efim A. Brener, Eran BouchbinderAbstract: The spontaneous nucleation of accelerating slip along slowly driven frictional interfaces is central to a broad range of geophysical, physical and engineering systems, with particularly far-reaching implications for earthquake physics. A common approach to this problem associates nucleation with an instability of an expanding creep patch upon surpassing a critical length Lc. The critical nucleation length Lc is conventionally obtained from a spring-block linear stability analysis extended to interfaces separating elastically-deformable bodies using model-dependent fracture mechanics estimates. We propose an alternative approach in which the critical nucleation length is obtained from a related linear stability analysis of homogeneous sliding along interfaces separating elastically-deformable bodies. For elastically identical half-spaces and rate-and-state friction, the two approaches are shown to yield Lc that features the same scaling structure, but with substantially different numerical pre-factors, resulting in a significantly larger Lc in our approach. The proposed approach is also shown to be naturally applicable to finite-size systems and bimaterial interfaces, for which various analytic results are derived. To quantitatively test the proposed approach, we performed inertial Finite-Element-Method calculations for a finite-size two-dimensional elastically-deformable body in rate-and-state frictional contact with a rigid body under sideway loading. We show that the theoretically predicted Lc and its finite-size dependence are in reasonably good quantitative agreement with the full numerical solutions, lending support to the proposed approach. These results offer a theoretical framework for predicting rapid slip nucleation along frictional interfaces.PubDate: 2017-11-02T17:00:27.65565-05:0DOI: 10.1002/2017GL074939

Authors:Wei Hu; Qiang Xu, Gonghui Wang, Gianvito Scaringi, Mauri McSaveney, Pierre-Yves HicherAbstract: We present results of ring-shear frictional resistance for mudstone granules of different size obtained from a landslide shear zone. Little rate dependency of shear resistance was observed in sand-sized granules in any wet or dry test, while saturated gravel-sized granules exhibited significant and abrupt reversible rate-weakening (from μ = 0.6 to 0.05) at about 2 mm/s. Repeating resistance variations occurred also under constant shear displacement rate. Mudstone granules generate mud as they are crushed and softened. Shear-thinning and thixotropic behavior of the mud can explain the observed behavior: with the viscosity decreasing, the mud can flow through the coarser soil pores and migrate out from the shear zone. This brings new granules into contact which produces new mud. Thus, the process can start over. Similarities between experimental shear zones and those of some landslides in mudstone suggest that the observed behaviour may play a role in some landslide kinematics.PubDate: 2017-10-31T18:26:20.690083-05:DOI: 10.1002/2017GL075261

Authors:Nick Dygert; Jung-Fu Lin, Edward W. Marshall, Yoshio Kono, James E. GardnerAbstract: Much of the lunar crust is monomineralic, comprising>98% plagioclase. The prevailing model argues the crust accumulated as plagioclase floated to the surface of a solidifying lunar magma ocean (LMO). Whether>98% pure anorthosites can form in a flotation scenario is debated. An important determinant of the efficiency of plagioclase fractionation is the viscosity of the LMO liquid, which was unconstrained. Here we present results from new experiments conducted on a late LMO-relevant ferrobasaltic melt. The liquid has an exceptionally low viscosity of 0.22−0.19+0.11 to 1.45−0.82+0.46 Pa·s at experimental conditions (1300-1600°C; 0.1-4.4GPa), and can be modeled by an Arrhenius relation. Extrapolating to LMO-relevant temperatures, our analysis suggests a low viscosity LMO would form a stratified flotation crust, with the oldest units containing a mafic component and with very pure younger units. Old, impure crust may have been buried by lower crustal diapirs of pure anorthosite in a serial magmatism scenario.PubDate: 2017-10-31T18:25:43.758786-05:DOI: 10.1002/2017GL075703

Authors:M. Broccardo; A. Mignan, S. Wiemer, B. Stojadinovic, D. GiardiniAbstract: In this study, we present a Bayesian hierarchical framework to model fluid-induced seismicity. The framework is based on a non-homogeneous Poisson process (NHPP) with a fluid-induced seismicity rate proportional to the rate of injected fluid. The fluid-induced seismicity rate model depends upon a set of physically meaningful parameters, and has been validated for six fluid-induced case studies. In line with the vision of hierarchical Bayesian modeling, the rate parameters are considered as random variables. We develop both the Bayesian inference and updating rules, which are used to develop a probabilistic forecasting model. We tested the Basel 2006 fluid-induced seismic case study to prove that the hierarchical Bayesian model offers a suitable framework to coherently encode both epistemic uncertainty and aleatory variability. Moreover, it provides a robust and consistent short-term seismic forecasting model suitable for online risk quantification and mitigation.PubDate: 2017-10-31T18:25:40.091189-05:DOI: 10.1002/2017GL075251

Authors:Natalia Artemieva; Joanna Morgan,Abstract: Potentially hazardous asteroids and comets have hit Earth throughout its history, with catastrophic consequences in the case of the Chicxulub impact. Here we reexamine one of the mechanisms that allow an impact to have a global effect—the release of climate-active gases from sedimentary rocks. We use the SOVA hydrocode and model ejected materials for a sufficient time after impact to quantify the volume of gases that reach high enough altitudes (> 25 km) to have global consequences. We vary impact angle, sediment thickness and porosity, water depth, and shock pressure for devolatilization and present the results in a dimensionless form so that the released gases can be estimated for any impact into a sedimentary target. Using new constraints on the Chicxulub impact angle and target composition, we estimate that 325 ± 130 Gt of sulfur and 425 ± 160 Gt CO2 were ejected and produced severe changes to the global climate.PubDate: 2017-10-30T16:00:01.975747-05:DOI: 10.1002/2017GL074879

Authors:Xiaolin Mao; Michael Gurnis, Dave A. MayAbstract: How subduction initiates with mechanically unfavorable lithospheric heterogeneities is important and rarely studied. We investigate this with a geodynamic model for the Puysegur Incipient Subduction Zone (PISZ) south of New Zealand. The model incorporates a true free surface, elasto-visco-plastic rheology and phase changes. Our predictions fit the morphology of the Puysegur Trench and Ridge and the deformation history on the overriding plate. We show how a new thrust fault forms and evolves into a smooth subduction interface, and how a preexisting weak zone can become a vertical fault inboard of the thrust fault during subduction initiation, consistent with two-fault system at PISZ. The model suggests that the PISZ may not yet be self-sustaining. We propose the Snares Zone (or Snares Trough) is caused by plate coupling differences between shallower and deeper parts, the tectonic sliver between two faults experiences strong rotation, and low density material accumulates beneath the Snares Zone.PubDate: 2017-10-30T13:55:41.431383-05:DOI: 10.1002/2017GL075389

Authors:A. C. A. Rudy; S. F. Lamoureux, S. V. Kokelj, I. R. Smith, J. H. EnglandAbstract: Recent climate warming has activated the melt-out of relict massive ice in permafrost-preserved moraines throughout the western Canadian Arctic. This ice that has persisted since the last glaciation, buried beneath as little as 1 m of overburden, is now undergoing accelerated permafrost degradation and thermokarst. Here we document recent and intensifying thermokarst activity on eastern Banks Island that has increased the fluvial transport of sediments and solutes to the ocean. Isotopic evidence demonstrates that a major contribution to discharge is melt of relict ground ice, resulting in a significant hydrological input from thermokarst augmenting summer runoff. Accelerated thermokarst is transforming the landscape and the summer hydrological regime, and altering the timing of terrestrial to marine and lacustrine transfers over significant areas of the western Canadian Arctic. The intensity of the landscape changes demonstrates that regions of cold, continuous permafrost are undergoing irreversible alteration, unprecedented since deglaciation (~13 cal ka BP).PubDate: 2017-10-30T13:55:26.716572-05:DOI: 10.1002/2017GL074912

Authors:E. R. Maúre; J. Ishizaka, C. Sukigara, Y. Mino, H. Aiki, T. Matsuno, H. Tomita, J. I. Goes, H. R. GomesAbstract: Satellite Chlorophyll a (CHL) data was used to investigate the influence of mesoscale anticyclonic eddies (AEs) and cyclonic eddies (CEs) on the timing of spring phytoplankton bloom initiation around the Yamato Basin (133-139° E and 35-39.5° N) in the Japan Sea, for the period 2002-2011. The results showed significant differences between AEs and CEs in the timing and initiation mechanism of the spring phytoplankton bloom. Blooms were initiated earlier in CEs which were characterized by shallow mixed-layer depths (< 100 m). The early blooming preceded the end of winter cooling (i.e., while net heat flux (Q0) is still negative) and is initiated by the increased average light within the shallow mixed-layer depth. Conversely, blooms appeared in the AEs despite deeper mixed-layer depth (> 100 m) but close to the commencement of positive Q0. This suggests that the relaxation of turbulent mixing is crucial for the bloom initiation in AEs.PubDate: 2017-10-30T13:55:23.959898-05:DOI: 10.1002/2017GL074359

Authors:Chuanjun Du; Zhiyu Liu, Shuh-Ji Kao, Minhan DaiAbstract: Nutrients from depth have been hypothesized as a primary source of new nutrients that sustain new productivity in oligotrophic oceans; however the flux is challenging to quantify. Here we show for a first time in the oligotrophic South China Sea an extremely low diapycnal dissolved inorganic nitrogen (DIN) flux as 1.8×10-4 mmol m-2 d-1 in the nutrient depleted layer (NDL) above the nutricline, and where other nutrient supplies sustain the new production. Here, higher phosphate and silicate fluxes relative to DIN than Redfield stoichiometry further indicate N-limited biological productivity and additional removal of DIN by diatoms. Below the NDL across the nutricline to the base of euphotic zone, termed as nutrient replete layer (NRL), the DIN flux is three orders of magnitude larger and sufficient in supporting the export production therein. Here, higher DIC flux relative to DIN than Redfield stoichiometry further infers DIC excess in the upper ocean.PubDate: 2017-10-26T17:42:31.802192-05:DOI: 10.1002/2017GL074921

Authors:Tae-Kyung Hong; Junhyung Lee, Donggeun Chi, Seongjun ParkAbstract: The 2011 Mw 9.0 Tohoku-Oki megathrust earthquake accompanied coseismic and postseismic displacements around the eastern Eurasian continental plate. Noise cross-correlations produced transient seismic waveforms along interstation paths in the Korean Peninsula. We measured the traveltime changes of the fundamental mode Rayleigh waves over the range of 0.03-0.08 Hz after the megathrust earthquake. The temporal seismic velocity changes in the lower crust were assessed from the traveltime changes. The traveltimes increased instantly after the megathrust earthquake and were gradually recovered over several hundreds to thousands of days. The instant shear-wave velocity decreases ranged between 0.731 (±0.057) and 4.068 (±0.173) %. The temporal medium perturbation might be caused by the transient uniaxial tensional stress due to the coseismic and postseismic displacements. The medium properties may be recovered by progressive stress field reconstruction.PubDate: 2017-10-26T17:42:28.28111-05:0DOI: 10.1002/2017GL075447

Authors:Wenju Cai; Guojian Wang, Agus Santoso, Xiaopei Lin, Lixin WuAbstract: During extreme El Niño events, the Intertropical Convergence Zone moves to the normally cold and dry east equatorial Pacific, resulting in a nonlinear rainfall increase with sea surface temperature in the region. An arbitrary threshold value of boreal winter total rainfall (e.g., 5 mm day-1) in the east equatorial Pacific was used by previous studies to capture this feature. Under greenhouse warming, the frequency of extreme El Niño events is projected to increase, so is the mean east equatorial Pacific rainfall. Is the projected frequency increase a consequence of the mean rainfall increase' We show that the projection is not significantly influenced by the increased mean rainfall. Instead, the increased frequency accounts for approximately 50% of the mean rainfall increase. Using upward atmospheric vertical velocity for defining extreme El Niño reaffirms the conclusion that the increased frequency results from increased probability of atmospheric deep convection, as the eastern equatorial Pacific warms faster than the surrounding regions.PubDate: 2017-10-26T17:41:55.199441-05:DOI: 10.1002/2017GL075635

Authors:Lin Qi; Chuanmin Hu, Mengqiu Wang, Shaoling Shang, Cara WilsonAbstract: A floating algae bloom in the East China Sea was observed in MODIS imagery in May 2017. Using satellite imagery from MODIS, VIIRS, GOCI, and OLI, and combined with numerical particle tracing experiments and laboratory experiments, we examined the history of this bloom as well as similar blooms in previous years, and attempted to trace the bloom source and identify the algae type. Results suggest that one bloom origin is offshore Zhejiang coast where algae slicks have appeared in satellite imagery almost every February – March since 2012. Following the Kuroshio Current and Taiwan Warm Current, these “initial” algae slicks are first transported to the northeast to reach South Korea (Jeju Island) and Japan coastal waters (up to 135oE) by early April 2017, and then transported to the northwest to enter the Yellow Sea by the end of April. The transport pathway covers an area known to be rich in Sargassum horneri, and spectral analysis suggests that most of the algae slicks may contain large amount of S. horneri. The bloom covers a water area of ~160,000 km2 with pure algae coverage of ~530 km2, which exceeds the size of most Ulva blooms that occur every May – July in the Yellow Sea. While blooms of smaller size also occurred in previous years and especially in 2015, the 2017 bloom is hypothesized to be a result of record-high water temperature, increased light availability, and continuous expansion of Porphyra aquaculture along the ECS coast.PubDate: 2017-10-26T17:41:53.435097-05:DOI: 10.1002/2017GL075525

Authors:Tatsuya Kubota; Tatsuhiko Saito, Wataru Suzuki, Ryota HinoAbstract: We examined the dynamic pressure change at the seafloor to estimate the centroid moment tensor solutions of the largest and second largest foreshocks (Mw 7.2 and 6.5) of the 2011 Tohoku-Oki earthquake. Combination of onshore broadband seismograms and high-frequency (~20–200 s) seafloor pressure records provided the resolution of the horizontal locations of the centroids, consistent with the results of tsunami inversion using the long-period (> ~10 min) seafloor pressure records although the depth was not constrained well, whereas the source locations were poorly constrained by the onshore seismic data alone. Also, the waveforms synthesized from the estimated CMT solution demonstrated the validity of the theoretical relationship between pressure change and vertical acceleration at the seafloor. The results of this study suggest that offshore pressure records can be utilized as offshore seismograms, which would be greatly useful for revealing the source process of offshore earthquakes.PubDate: 2017-10-26T17:41:43.121599-05:DOI: 10.1002/2017GL075386

Authors:Quentin Bletery; Amanda M. Thomas, Alan W. Rempel, Jeanne L. HardebeckAbstract: Subduction faults accumulate stress during long periods of time and release this stress suddenly, during earthquakes, when it reaches a threshold. This threshold, the shear strength, controls the occurrence and magnitude of earthquakes. We consider a 3D model to derive an analytical expression for how the shear strength depends on the fault geometry, the convergence obliquity, frictional properties and the stress field orientation. We then use estimates of these different parameters in Japan to infer the distribution of shear strength along a subduction fault. We show that the 2011 Mw9.0 Tohoku earthquake ruptured a fault portion characterized by unusually small variations in static shear strength. This observation is consistent with the hypothesis that large earthquakes preferentially rupture regions with relatively homogeneous shear strength. With increasing constraints on the different parameters at play, our approach could, in the future, help identify favorable locations for large earthquakes.PubDate: 2017-10-26T17:41:39.525297-05:DOI: 10.1002/2017GL075501

Authors:Z. H. Yao; A. Radioti, I. J. Rae, J. Liu, D. Grodent, L. C. Ray, S. V. Badman, A. J. Coates, J.-C. Gérard, J. H. Waite, J. N. Yates, Q. Q. Shi, Y. Wei, B. Bonfond, M. K. Dougherty, E. Roussos, N. Sergis, B. PalmaertsAbstract: Although auroral emissions at giant planets have been observed for decades, the physical mechanisms of aurorae at giant planets remain unclear. One key reason is the lack of simultaneous measurements in the magnetosphere while remote sensing of the aurora. We report a dynamic auroral event identified with the Cassini Ultraviolet Imaging Spectrograph (UVIS) at Saturn on 13 July 2008 with coordinated measurements of the magnetic field and plasma in the magnetosphere. The auroral intensification was transient, only lasting for ∼ 30 minutes. The magnetic field and plasma are perturbed during the auroral intensification period. We suggest that this intensification was caused by wave mode conversion generated field-aligned currents, and we propose two potential mechanisms for the generation of this plasma wave and the transient auroral intensification. A survey of the Cassini-UVIS database reveals that this type of transient auroral intensification is very common (10/11 time sequences, and ∼10% of the total images).PubDate: 2017-10-26T17:41:16.631964-05:DOI: 10.1002/2017GL075108

Authors:Yi Hu; Boris Kiefer, Craig R. Bina, Dongzhou Zhang, Przemyslaw K. DeraAbstract: In situ X-ray diffraction experiments with natural Fe- and Al- bearing diopside single crystals and density functional theory (DFT) calculations on diopside end-member composition indicate the existence of a new high-pressure γ-diopside polymorph with rare penta-coordinated silicon. On compression α-diopside transforms to the γ-phase at ∼ 50 GPa, which in turn, on decompression is observed to convert to the known β-phase below 47 GPa. The new γ-diopside polymorph constitutes another recent example of penta-coordinated silicon (VSi) in over-compressed metastable crystalline silicates, suggesting that VSi may exist in the transition zone and the uppermost lower mantle in appreciable quantities, not only in silicate glass and melts, but also in crystalline phases contained in the coldest parts of subducted stagnant slabs. VSi may have significant influences on buoyancy, wave velocity anomalies, deformation mechanisms, chemical reactivity of silicate rocks and seismicity within the slab.PubDate: 2017-10-26T17:41:02.766877-05:DOI: 10.1002/2017GL075424

Authors:Chunquan Wu; Eric G. DaubAbstract: Tectonic tremor and low-frequency earthquakes (LFE) have been identified globally, but the physical mechanisms responsible for different types of LFE recurrence patterns are still elusive. Here we use a brittle ductile friction model to study the frictional conditions leading to different LFE recurrence patterns observed in central California. We do a comprehensive search of the friction parameters, including the brittle contact failure length and ductile damping strength to generate synthetic LFE bursts that match three different occurrence patterns of LFE bursts. We find that the output synthetic LFE burst recurrence type varies under different frictional conditions. The chaotic recurrence LFE sources require a higher brittle-to-ductile friction ratio, consistent with the observation that they are mainly at shallower part of the brittle-ductile transition zone. The tri-modal recurrence LFE sources require a wide distribution of asperity sizes, indicating that the segment of deep SAF between Monarch Peak and Parkfield is likely to be more fragmented and weaker than the surrounding segments.PubDate: 2017-10-26T17:40:26.26365-05:0DOI: 10.1002/2017GL075402

Authors:Muchamad Al Azhar; Zouhair Lachkar, Marina Lévy, Shafer SmithAbstract: The combination of high primary productivity and weak ventilation in the Arabian Sea (AS) and Bay of Bengal (BoB) generates vast areas of depleted oxygen, known as Oxygen Minimum Zones (OMZs). The AS OMZ is the world's thickest and hosts up to 40% of global denitrification. In contrast, the OMZ in the BoB is weaker and denitrification free. Using a series of model simulations, we show that the deeper remineralization depth (RD) in the BoB, potentially associated with organic matter aggregation with riverine mineral particles, contributes to weaken its OMZ. When the RD is set uniformly across both seas, the model fails to reproduce the observed contrast between the two OMZs, irrespective of the chosen RD. In contrast, when the RD is allowed to vary spatially, the contrasting distributions of oxygen and nitrate are correctly reproduced, and water column denitrification is simulated exclusively in the AS, in agreement with observations.PubDate: 2017-10-23T21:11:14.822969-05:DOI: 10.1002/2017GL075157

Authors:C. Holden; Y. Kaneko, E. D’Anastasio, R. Benites, B. Fry, I. J. HamlingAbstract: The 2016 Kaikōura (New Zealand) earthquake generated large ground motions and resulted in multiple onshore and offshore fault ruptures, a profusion of triggered landslides, and a regional tsunami. Here we examine the rupture evolution using two kinematic modelling techniques based on analysis of local strong-motion and high-rate GPS data. Our kinematic models capture a complex pattern of slowly (Vr < 2km/s) propagating rupture from the south to north, with over half of the moment release occurring in the northern source region, mostly on the Kekerengu fault, 60 seconds after the origin time. Both models indicate rupture re-activation on the Kekerengu fault with the time separation of ~11 seconds between the start of the original failure and start of the subsequent one. We further conclude that most near-source waveforms can be explained by slip on the crustal faults, with little (PubDate: 2017-10-23T21:10:46.829935-05:DOI: 10.1002/2017GL075301

Authors:B. Gerling; H. Löwe, A. van HerwijnenAbstract: The elastic modulus is the most fundamental mechanical property of snow. However literature values scatter by orders of magnitude and hitherto no cross-validated measurements exists. To this end we employ P-wave propagation experiments under controlled laboratory conditions on decimeter-sized snow specimen, prepared from artificial snow and subjected to isothermal sintering, to cover a considerable range of densities (170 − 370kgm−3). The P-wave modulus was estimated from wave propagation speeds in transverse isotropic media and compared to microstructure-based finite element (FE) calculations from X-ray tomography images. Heterogeneities and size differences between acoustic and FE sample volumes were characterized by SnowMicroPen measurements, yielding an elastic modulus as a by-product. The moduli (10 − 340MPa) from the acoustic and FE method are in very good agreement (R2=0.99) over the entire range of densities. A remaining bias (24 %) between both methods can be explained by layer heterogeneities which systematically reduce the estimates from the acoustic method.PubDate: 2017-10-23T21:10:36.056231-05:DOI: 10.1002/2017GL075110

Authors:Roger P. Leyser; Suzanne M. Imber, Steve E. Milan, James A. SlavinAbstract: Analysis of MESSENGER data has shown for the first time that the orientation of the Interplanetary Magnetic Field (IMF) in the magnetosheath of Mercury plays a crucial role in the formation of flux transfer events (FTEs) at the dayside magnetopause. During the first 4 Hermean years of MESSENGER's orbit around Mercury, we have identified 805 FTEs using magnetometer data. Under conditions of near-southward IMF, at least one FTE was detected on nearly 70% of passes through the magnetopause but the observation rate during northward IMF was less than 20%. FTEs were also observed preferentially in the pre-noon sector.PubDate: 2017-10-23T21:10:29.954322-05:DOI: 10.1002/2017GL074858

Authors:Y. Choi; M. Morlighem, E. Rignot, J. Mouginot, M. WoodAbstract: Recent studies have shown that the NorthEast Greenland Ice Sheet region (NEGIS) has been undergoing significant acceleration and dynamic thinning since 2010 and these changes are closely related to regional atmospheric and oceanic warming. Here we model the response of Nioghalvfjerdsfjorden (79North) and Zachariae Isstrøm (ZI) to ocean forcing to investigate their evolution over the coming decades. Our model suggests that 79North will retreat slowly over the next century, whereas ZI will lose its floating ice tongue completely and retreat rapidly for 70 years. After 70 years, ZI will stabilize 30 km upstream of its current position on a topographic ridge. Frontal melt rates need to reach 6 m/day in the summer to dislodge the glacier from this ridge. ZI will then continue a fast and unstoppable retreat, contributing more than 16.2 mm to global sea level rise by 2100.PubDate: 2017-10-23T21:10:13.63986-05:0DOI: 10.1002/2017GL075174

Authors:Ian J. HamlingAbstract: Many volcanoes around the world host summit crater lakes but their influence on the overall stability of the edifice remains poorly understood. Here I use satellite radar data acquired by TerraSAR-X from early 2015 to July 2017 over White Island, New Zealand, to investigate the interaction of the crater lake and deformation of the surrounding edifice. An eruption in April 2016 was preceded by a period of uplift within the crater floor and drop in the lake level. Modeling of the uplift indicates a shallow source located at ∼100 m depth in the vicinity of the crater lake, likely coinciding with the shallow hydrothermal system. In addition to the drop in the lake level, stress changes induced by the inflation suggest that the pressurization of the shallow hydrothermal system helped promote failure along the edge of the crater lake which collapsed during the eruption. After the eruption, and almost complete removal of the crater lake, large areas of the crater wall and lake edge began moving downslope at rates approaching 400 mm/yr. The coincidence between the rapid increase in the displacement rates and removal of the crater lake suggests that the lake provides a physical control on the stability of the surrounding edifice.PubDate: 2017-10-23T21:08:23.914426-05:DOI: 10.1002/2017GL075572

Authors:Magali I BillenAbstract: Backarc spreading centers initiate as forearc or arc rifting events when extensional forces localize within lithosphere weakened by hydrous fluids or melting. Two models have been proposed for triggering forearc/arc rifting: roll-back of the subducting plate causing trench retreat, or motion of the overriding plate away from the subduction zone. This paper demonstrates that there is a third mechanism caused by an in situ instability that occurs when the thin high-viscosity boundary, which separates the weak forearc from the hot buoyant mantle wedge, is removed. Buoyant upwelling mantle causes arc rifting, drives the overriding plate away from the subducting plate, and there is sufficient heating of the subducting plate crust and overriding plate lithosphere to form adakite or boninite volcanism. For spontaneous forearc/arc rifting to occur a broad region of weak material must be present and one of the plates must be free to respond to the upwelling forces.PubDate: 2017-10-23T21:06:30.117564-05:DOI: 10.1002/2017GL075061

Authors:L. P. BabichAbstract: In view of the neutron flux enhancements observed in thunderstorms, a contribution of thunderstorm neutrons to atmospheric radiocarbon (isotope C614) production is analyzed in connection with the archaeometry. Herein, estimates of neutron fluence per lightning electromagnetic pulse in regions with severe thunderstorm activity, at which a local rate of the C614 production is comparable to the observed rates, are shown to be consistent with the measured magnitudes of thunderstorm neutron fluence. At present, available observations of atmospheric neutron and parent gamma-ray flashes correlated with thunderstorms do not allow making final conclusions about thunderstorm contributions to C614 production. For this, numerous studies of high-energy phenomena in thunderstorms are required, especially in the tropical belt where the thunderstorm activity is especially severe and where the C614 production by galactic cosmic rays is almost independent of the solar activity disturbing the Earth's magnetic field shielding the Earth from cosmic rays.PubDate: 2017-10-23T21:05:42.019464-05:DOI: 10.1002/2017GL075131

Authors:S. Kaboli; P. C. Burnley, G. Xia, H. W. GreenAbstract: We present measurements of the creep strength of forsterite olivine at 1200 °C from 1- 7.5 GPa made using a Griggs apparatus at low pressures and a D-DIA apparatus at high pressures. The same starting material is used for all experiments allowing us to compare measurements made with the two instruments. We find that results from the two apparatus are comparable if the stress measurements made from the alumina piston in the D-DIA experiments are used as a proxy for the load that the olivine sample supports. The reproducibility of the D-DIA experiments are improved if microstructural observations of the post experimental integrity of the sample and piston are used to judge experimental validity. Interpretation of the joint dataset indicate that the pressure dependence of creep (ΔV*) for our sample material cannot be described by a constant value over this pressure range, but changes from ~15 cm3/mol at low pressure to closer to ~4 cm3/mol at high pressure. These results are consistent with changes in the mix of deformation mechanisms operating in forsterite as pressure increases.PubDate: 2017-10-20T20:12:20.176131-05:DOI: 10.1002/2017GL075177

Authors:Zhuo Ye; Jiangtao Li, Rui Gao, Xiaodong Song, Qiusheng Li, Yingkang Li, Xiao Xu, Xingfu Huang, Xiaosong Xiong, Wenhui LiAbstract: Based on a dense linear seismic array traversing the eastern margin of the Tibetan Plateau into the Qinling Belt, we conducted a joint inversion of receiver functions and surface wave dispersion curves under P-wave velocity constraints and simultaneously derived a crustal and uppermost mantle Vs profile with a Vp/Vs profile. Our observations indicate that the Qinling Belt, of which the lower crust exhibits Vp/Vs values less than 1.8 that are indicative of an intermediate to felsic composition, is currently not serving as a channel accommodating the extrusion of mid-lower crustal materials of the Tibetan Plateau. Channelized ductile mantle flow from beneath the Tibetan Plateau through the Qinling Belt would be feasible only at sub-lithospheric depths (i.e., in the asthenosphere). Our results suggest that the extrusion of ductile mid-lower crustal materials accompanied by fault-related tectonics and isostatic buoyancy resulting from lithospheric detachment (triggered by asthenospheric flow) may have jointly engendered the plateau uplift and expansion in the Tibet-Qinling transition zone.PubDate: 2017-10-20T20:11:22.937864-05:DOI: 10.1002/2017GL075141

Authors:Huilin Wang; Michael Gurnis, Jakob SkogseidAbstract: Enigmatic surface deflections occurred in North America starting from the Cretaceous, including the continental-scale drainage reorganization and the long-wavelength subsidence in the Western Interior Seaway. These surface undulations cannot be simply explained by sea-level change or flexure loading. Coinciding with the large-scale surface deflection, the Gulf of Mexico (GOM) has an immense Paleocene sediment deposition probably caused by tectonic subsidence. Increasing evidence indicates a distinct seismic anomaly localized in the mantle below the GOM. With geodynamic models, we show that the Hess Rise conjugate coincides with the position of the seismic anomaly. The basalt-eclogite transition in the Hess conjugate can lead to a localized dynamic subsidence in the GOM, which is superimposed on the broad surface deflection caused by the Farallon slab. The Hess conjugate, transformed to eclogite, could tilt the surface southward in the U.S. and help frame the GOM as a main depocenter in the Cenozoic.PubDate: 2017-10-20T20:11:06.801533-05:DOI: 10.1002/2017GL074959

Authors:C. L. da Silva; R. M. Millan, D. G. McGaw, C. T. Yu, A. S. Putter, J. LaBelle, J. DwyerAbstract: We provide extensive evidence that runaway electron acceleration and subsequent bremsstrahlung X-ray emission are a common feature in negative electrical discharges with voltages as low as 100 kV, indicating that all negative lightning could potentially produce runaway electrons. Centimeter-long streamer corona discharges produce bursts of X-ray radiation, emitted by a source highly compact in space and time, leading to photon pileup. Median photon burst energies vary between 33-96 keV in 100-kV discharges. Statistical analysis of 5000+ discharges shows that X-rays are observed in as many as 60% of the triggers, depending on the configuration. X-ray detection is more frequent when: streamers are not followed by a spark, the detector is oriented perpendicular to the gap, and a thicker anode is used. In an 8-cm-long gap, X-rays are produced when runaway electrons hit the anode, and the electron acceleration is not necessarily correlated with streamer collisions.PubDate: 2017-10-19T19:56:08.109866-05:DOI: 10.1002/2017GL075262

Authors:Maureen L. Nietiadi; Philipp Umstätter, Iyad Alabd Alhafez, Yudi Rosandi, Eduardo M. Bringa, Herbert M. UrbassekAbstract: Collisions between ice grains are ubiquitous in the outer solar system. The mechanics of such collisions is traditionally described by the elastic contact theory of adhesive spheres. Here we use molecular dynamics simulations to study collisions between nanometer-sized amorphous water-ice grains. We demonstrate that the collision-induced heating leads to grain melting in the interface of the colliding grain. The large lateral deformations and grain sticking induced considerably modify available macroscopic collision models. We report on systematic increases of the contact radius, strong grain deformations, and the prevention of grain bouncing.PubDate: 2017-10-19T19:56:02.465582-05:DOI: 10.1002/2017GL075395

Authors:Louis De Barros; Anne Deschamps, Anthony Sladen, Hélène Lyon-Caen, Nicholas VoulgarisAbstract: Dynamic triggering has been commonly observed after large teleseismic events, but the physics behind it is still under debate. To broaden observations, we here focus on the dynamic triggering by regional earthquakes, i.e. by events with magnitude lower than 6.2 at distances smaller than 600 km. The western part of the Corinth Rift (Greece) is characterized by intense seismic swarms, and is therefore adapted to study such responses. The microseismicity rates before and after the transient perturbations are high enough to analyze 30 regional earthquakes out of the 59 occurring in 2013. More than 40 % of those 30 events, including earthquakes with magnitude as small as 4.5, are associated with a significant seismicity rate increase. The triggerability primarily depends on the amplitude of the seismic waves. However, triggering is mainly observed when the seismic perturbations are orthogonal to the faults, which suggests that fluid pressurization is likely involved.PubDate: 2017-10-18T17:25:45.343843-05:DOI: 10.1002/2017GL075460

Authors:Kevin M. Befus; Kevin D. Kroeger, Christopher G. Smith, Peter W. SwarzenskiAbstract: Fresh groundwater discharge to coastal environments contribute to the physical and chemical conditions of coastal waters, but the roles of coastal groundwater at regional to continental scales remains poorly defined due to diverse hydrologic conditions and the difficulty of tracking coastal groundwater flow paths through heterogeneous subsurface materials. We use three-dimensional groundwater flow models for the first time to calculate the magnitude and source areas of groundwater discharge from unconfined aquifers to coastal waterbodies along the entire eastern U.S. We find 27.1 km3/yr (22.8-30.5 km3/yr) of groundwater directly enters eastern U.S. and Gulf of Mexico coastal waters. The contributing recharge areas comprised ~175,000 km2 of U.S. land area, extending several kilometers inland. This result provides new information on the land area that can supply natural and anthropogenic constituents to coastal waters via groundwater discharge, thereby defining the subterranean domain potentially affecting coastal chemical budgets and ecosystem processes.PubDate: 2017-10-18T17:25:41.820398-05:DOI: 10.1002/2017GL075238

Authors:A. Okamoto; H. Takana, N. Watanabe, H. Saishu, N. TsuchiyaAbstract: Fractures are the location of various water–rock interactions within the Earth's crust; however, the impact of the chemical heterogeneity of fractures on hydraulic properties is poorly understood. We conducted flow-through experiments on the dissolution of granite with a tensile fracture at 350°C and fluid pressure of 20 MPa with confining pressure of 40 MPa. The aperture structures were evaluated by X-ray computed tomography (CT) before and after the experiments. Under the experimental conditions, quartz grains dissolve rapidly to produce grain-scale pockets on the fracture surface; whereas altered feldspar grains act as asperities to sustain the open cavities. The fracture contained gouge with large surface area. The feedback between fluid flow and the rapid dissolution of gouge material produced large fluid pockets, whereas permeability did not always increase significantly. Such intense hydrological–chemical interactions could strongly influence the porosity-permeability relationship of fractured reservoirs in the crust.PubDate: 2017-10-18T17:25:37.934062-05:DOI: 10.1002/2017GL075476

Authors:J. H. LaCasceAbstract: Satellite observations have revolutionized oceanography, capturing diverse phenomena over much of the globe. However, it remains to understand how surface fields, like sea surface height, reflect the motion occurring at depth. The vertical structure of ocean eddies is often expressed in terms of “baroclinic modes”, which are basis functions derived assuming a flat ocean bottom. Bathymetry alters the modes though, weakening the bottom velocities. Using analytical solutions, we demonstrate that with realistic bathymetry and/or bottom friction, the bottom velocities are nearly zero. The resulting “surface modes” should be ubiquitous in the ocean. This would explain the dominant mode of variability obtained from globally-distributed current meter data, and is consistent with energy spectra derived from sea surface height data. The results yield a simple way to infer subsurface velocities from satellite data, and suggest ocean analyses should be made in terms of surface modes and topographic waves.PubDate: 2017-10-18T17:20:34.777427-05:DOI: 10.1002/2017GL075430

Authors:Max D. Holloway; Louise C. Sime, Claire S. Allen, Claus-Dieter Hillenbrand, Pete Bunch, Eric Wolff, Paul J. ValdesAbstract: We compare multi-ice core data with δ18O model output for the early last interglacial Antarctic sea-ice minimum. The spatial pattern of δ18O across Antarctica is sensitive to the spatial pattern of sea-ice retreat. Local sea ice retreat increases the proportion of winter precipitation, depleting δ18O at ice core sites. However, retreat also enriches δ18O because of the reduced source-to-site distance for atmospheric vapour. The joint overall effect is for δ18O to increase as sea ice is reduced.ÂăOur data-model comparison indicates a winter sea-ice retreat of 67, 59 and 43 % relative to pre-industrial in the Atlantic, Indian and Pacific sectors of the Southern Ocean. A compilation of Southern Ocean sea-ice proxy data provides weak support for this reconstruction. However, most published marine core sites are located too far north of the 128,000 years BP sea ice edge, preventing independent corroboration for this sea ice reconstruction.PubDate: 2017-10-18T17:15:46.956408-05:DOI: 10.1002/2017GL074594

Authors:R. Niraula; T. Meixner, F. Dominguez, N. Bhattarai, M. Rodell, H. Ajami, D. Gochis, C. CastroAbstract: Although groundwater is a major water resource in the western US, little research has been done on the impacts of climate change on groundwater storage and recharge in the West. Here we assess the impact of projected changes in climate on groundwater recharge in the near (2021-2050) and far (2071-2100) future across the western US. VIC model was run with RCP 6.0 forcing from 11 GCMs and ‘subsurface runoff’ output was considered as recharge. Recharge is expected to decrease in the West (-5.8±14.8 %) and Southwest (-4.0±6.7%) regions in the near future and in the South region (-9.5±24.3%) in the far future. The Northern Rockies region is expected to get more recharge in the near (+5.3±6.3%) and far (+11.8±8.3%) future. Overall, southern portions of the western US are expected to get less recharge in the future and northern portions will get more. Climate change interacts with land surface properties to affect the amount of recharge that occurs in the future. Effects on recharge due to change in vegetation response from projected changes in climate and CO2 concentration, though important, are not considered in this study.PubDate: 2017-10-16T13:35:30.221385-05:DOI: 10.1002/2017GL075421

Authors:B. de Boer; Alan M. Haywood, Aisling M. Dolan, Stephen J. Hunter, Caroline L. PrescottAbstract: Examining the nature of ice-sheet and sea-level response to past episodes of enhanced greenhouse gas forcing may help constrain future sea-level change. Here, for the first time, we present the transient nature of ice sheets and sea level during the Late Pliocene. The transient ice-sheet predictions are forced by multiple climate snapshots derived from a climate model set up with Late Pliocene boundary conditions, forced with different orbital forcing scenarios appropriate to two Marine Isotope Stages (MIS), MIS KM5c and K1. Our results indicate that during MIS KM5c both the Antarctic and Greenland ice sheets contributed to sea-level rise relative to present and were relatively stable. Insolation forcing between the hemispheres was out of phase during MIS K1 and led to an asynchronous response of ice volume globally. Therefore, when variations of precession were high, inferring the behavior of ice sheets from benthic isotope or sea-level records is complex.PubDate: 2017-10-16T13:31:28.39295-05:0DOI: 10.1002/2017GL073535

Authors:C. Ole Wulff; Richard J. Greatbatch, Daniela I.V. Domeisen, Gereon Gollan, Felicitas HansenAbstract: The Summer East Atlantic (SEA) mode is the second dominant mode of summer low-frequency variability in the Euro-Atlantic region. Using reanalysis data, we show that SEA-related circulation anomalies significantly influence temperatures and precipitation over Europe. We present evidence that part of the interannual SEA variability is forced by diabatic heating anomalies of opposing signs in the tropical Pacific and Caribbean that induce an extratropical Rossby wave train. This precipitation dipole is related to SST anomalies characteristic of the developing ENSO phases. Seasonal hindcast experiments forced with observed sea surface temperatures (SST) exhibit skill at capturing the interannual SEA variability corroborating the proposed mechanism and highlighting the possibility for improved prediction of boreal summer variability. Our results indicate that tropical forcing of the SEA likely played a role in the dynamics of the 2015 European heat wave.PubDate: 2017-10-16T13:30:52.236269-05:DOI: 10.1002/2017GL075493

Authors:Roohi P. S. Ghelani; Eric C. J. Oliver, Neil J. Holbrook, Matthew C. Wheeler, Philip J. KlotzbachAbstract: Rainfall in tropical Australia is a critical resource for the agricultural sector. However, its high variability implores improvements in our understanding of its variability. Australian tropical rainfall is influenced by both the Madden-Julian Oscillation (MJO) on intraseasonal time scales and El Niño-Southern Oscillation (ENSO) on interannual time scales. This study examines the joint relationship between the MJO, ENSO and tropical Australian rainfall variability. We analyse daily precipitation data from stations across tropical Australia during the wet season (November to April). The wet season rainfall response to the MJO is found to be greater during El Niño than La Niña. We demonstrate that this relationship is not due to the statistical relationship between the MJO and ENSO indices but instead due to differences in how the MJO modulates the large scale circulation during El Niño versus during La Niña.PubDate: 2017-10-16T13:30:39.172304-05:DOI: 10.1002/2017GL075452

Authors:M. Diallo; F. Ploeger, P. Konopka, T. Birner, R. Müller, M. Riese, H. Garny, B. Legras, E. Ray, G. Berthet, F. JegouAbstract: The stratospheric circulation is an important element of climate as it determines the concentration of radiatively active species like water vapour and aerosol above the tropopause. Climate models predict that increasing greenhouse gas levels speed up the stratospheric circulation. However, these results have been challenged by observational estimates of the circulation strength, constituting an uncertainty in current climate simulations. Here, we quantify the effect of volcanic aerosol on the stratospheric circulation focussing on the Mt. Pinatubo eruption and discussing further the minor extratropical volcanic eruptions after 2008. We show that the observed pattern of decadal circulation change over the past decades is substantially driven by volcanic aerosol injections. Thus, climate model simulations need to realistically take into account the effect of volcanic eruptions, including the minor eruptions after 2008, for a reliable reproduction of observed stratospheric circulation changes.PubDate: 2017-10-16T13:30:26.252738-05:DOI: 10.1002/2017GL074662

Authors:René M. van Westen; Henk A. DijkstraAbstract: Analysis of model data from a long (200 years) simulation of a high-resolution version of the Parallel Ocean Program indicates a connection between a mode of multidecadal variability in the Southern Ocean, the so-called Southern Ocean Mode, and multidecadal variability in the North Brazil Current. The multidecadal sea surface height variability in the Southern Ocean, propagates northwards, and submerges at about 40°S. Northward propagating anomalies in ocean heat content are found between 5°– 40°S at depths down to 1 km and affect the North Brazil Current. Similar variability and connections between Southern Ocean and North Brazil Current are also found in a (200 years) simulation of a high-resolution global version of the Community Earth System Model. The results provide a new mechanism for the low-frequency variability of the North Brazil Current.PubDate: 2017-10-16T13:26:05.603174-05:DOI: 10.1002/2017GL074815

Authors:Kuai Fang; Chaopeng Shen, Daniel Kifer, Xiao YangAbstract: The Soil Moisture Active Passive (SMAP) mission has delivered valuable sensing of surface soil moisture since 2015. However, it has a short time span and irregular revisit schedules. Utilizing a state-of-the-art time-series deep learning neural network, Long Short-Term Memory (LSTM), we created a system that predicts SMAP level-3 moisture product with atmospheric forcings, model-simulated moisture, and static physiographic attributes as inputs. The system removes most of the bias with model simulations and improves predicted moisture climatology, achieving small test root-mean-squared errors (0.87 for over 75% of Continental United States, including the forested Southeast. As the first application of LSTM in hydrology, we show the proposed network avoids overfitting and is robust for both temporal and spatial extrapolation tests. LSTM generalizes well across regions with distinct climates and environmental settings. With high fidelity to SMAP, LSTM shows great potential for hindcasting, data assimilation, and weather forecasting.PubDate: 2017-10-16T13:25:40.114339-05:DOI: 10.1002/2017GL075619

Authors:Patrik L. Pfister; Thomas F. StockerAbstract: Growing evidence from General Circulation Models (GCMs) indicates that the equilibrium climate sensitivity (ECS) depends on the magnitude of forcing, which is commonly referred to as state-dependence. We present a comprehensive assessment of ECS state-dependence in Earth System Models of Intermediate Complexity (EMICs) by analyzing millennial simulations with sustained 2×CO2 and 4×CO2 forcings. We compare different extrapolation methods and show that ECS is smaller in the higher-forcing scenario in 12 out of 15 EMICs, in contrast to the opposite behavior reported from GCMs. In one such EMIC, the Bern3D-LPX model, this state-dependence is mainly due to the weakening sea ice-albedo feedback in the Southern Ocean, which depends on model configuration. Due to ocean mixing adjustments, state-dependence is only detected hundreds of years after the abrupt forcing, highlighting the need for long model integrations. Adjustments to feedback parametrizations of EMICs may be necessary if GCM intercomparisons confirm an opposite state-dependence.PubDate: 2017-10-16T13:25:21.659028-05:DOI: 10.1002/2017GL075457

Authors:Xian Lu; Xinzhao Chu, Cao Chen, Vu Nguyen, Anne K. SmithAbstract: Unique Fe lidar observations in May 2014 at McMurdo, combined with Aura-Microwave Limb Sounder (MLS) measurements, lead to a new discovery that the amplitudes of 4-day and 2.5-day planetary waves (PWs) grow rapidly from 1–2 K at 100 km to over 10 K at 110 km. This report is also the first observation of short-period (1–5 days) eastward propagating PWs from 30 km all the way to 110 km. The Specified Dynamics-Whole Atmosphere Community Climate Model reproduces the observed three dominant peaks of amplitudes in temperature and coherent vertical phase structures. The data-model comparison indicates a possible mechanism: After PWs originated from the stratosphere dissipate along the critical level, the surviving waves are amplified by in-situ instability in the mesosphere and lower thermosphere, resulting in the second (third) peak in geopotential (temperature). This third peak in temperature explains the PW amplitude growth from 100 to 110 km.PubDate: 2017-10-13T19:05:43.153991-05:DOI: 10.1002/2017GL075641

Authors:Ioanna Merkouriadi; Bin Cheng, Robert M. Graham, Anja Rösel, Mats A. GranskogAbstract: During the Norwegian young sea ICE (N-ICE2015) campaign in early 2015, a deep snow pack was observed, almost double the climatology for the region north of Svalbard. There were significant amounts of snow-ice in second-year ice (SYI), while much less in first-year ice (FYI). Here we use a 1-D snow/ice thermodynamic model, forced with reanalyses, to show that snow-ice contributes to thickness growth of SYI in absence of any bottom growth, due to the thick snow. Growth of FYI is tightly controlled by the timing of growth onset relative to precipitation events. A later growth-onset can be favorable for FYI growth due to less snow accumulation, which limits snow-ice formation. We surmise these findings are related to a phenomenon in the Atlantic sector of the Arctic, where frequent storm events bring heavy precipitation during autumn and winter, in a region with a thinning ice cover.PubDate: 2017-10-12T18:35:30.859368-05:DOI: 10.1002/2017GL075494

Authors:J. H. Yee; J. Gjerloev, D. Wu, M. J. SchwartzAbstract: Using the O2 118 GHz spectral radiance measurements obtained by the Microwave Limb Sounder (MLS) instrument onboard the Aura spacecraft, we demonstrate that the Zeeman effect can be used to remotely measure the magnetic field perturbations produced by the auroral electrojet near the Hall current closure altitudes. Our derived current-induced magnetic field perturbations are found to be highly correlated with those coincidently obtained by ground magnetometers. These perturbations are also found to be linearly correlated with auroral electrojet strength. The statistically-derived polar maps of our measured magnetic field perturbation reveal a spatial-temporal morphology consistent with that produced by the Hall current during substorms and storms. With today's technology, a constellation of compact, low-power, high-spectral-resolution cubesats would have the capability to provide high precision and spatiotemporal magnetic-field samplings needed for auroral electrojet measurements to gain insights into the spatiotemporal behavior of the auroral electrojet system.PubDate: 2017-10-12T18:35:24.683373-05:DOI: 10.1002/2017GL074909

Authors:S. Ruiz; F. Aden-Antoniow, J. C. Baez, C. Otarola, B. Potin, F. Campo, P. Poli, C. Flores, C. Satriano, F. Leyton, R. Madariaga, P. BernardAbstract: The Valparaiso 2017 sequence occurred in the Central Chile mega-thrust, an active zone where the last mega-earthquake occurred in 1730. Intense seismicity started 2 days before the Mw 6.9 main-shock, a slow trench-ward movement was observed in the coastal GPS antennas and was accompanied by foreshocks and repeater-type seismicity. To characterize the rupture process of the main-shock, we perform a dynamic inversion using the strong-motion records and an elliptical patch approach. We suggest that a slow slip event preceded and triggered the Mw 6.9 earthquake, which ruptured an elliptical asperity (semi-axis of 10 km and 5 km, with a sub-shear rupture, stress drop of 11.71 MPa, yield stress of 17.21 MPa, slip weakening of 0.65 m and kappa value of 1.98). This earthquake could be the beginning of a long-term nucleation phase to a major rupture, within the highly coupled Central Chile zone where a mega-thrust earthquake like 1730 is expected.PubDate: 2017-10-12T18:25:22.06504-05:0DOI: 10.1002/2017GL075675

Authors:Laura Landrum; Marika Holland, Marilyn Raphael, Lorenzo PolvaniAbstract: It has been suggested that recent regional trends in Antarctic sea ice might have been caused by the formation of the ozone hole in the late 20th century. Here we explore this by examining two ensembles of a climate model over the ozone hole formation period (1955-2005). One ensemble includes all known historical forcings; the other is identical except for ozone levels, which are fixed at 1955 levels. We demonstrate that the model is able to capture, on interannual and decadal time scales, the observed statistical relationship between summer Amundsen Sea Low strength (when ozone loss causes a robust deepening) and fall sea-ice concentrations (when observed trends are largest). In spite of this, the modeled regional trends caused by ozone depletion are found to be almost exactly opposite to the observed ones. We deduce that the regional character of observed sea ice trends is likely not caused by ozone depletion.PubDate: 2017-10-11T14:53:59.224565-05:DOI: 10.1002/2017GL075618

Authors:Furrukh Bashir; Xubin Zeng, Hoshin Gupta, Pieter HazenbergAbstract: Glaciers in the eastern Hindukush, western Karakoram and northwestern Himalayan mountain ranges of Northern Pakistan are not responding to global warming in the same manner as their counterparts elsewhere. Their retreat rates are less than the global average, and some are either stable or growing. Various investigations have questioned the role of climatic factors in regards to this anomalous behavior, widely referred to as ‘The Karakoram Anomaly’. Here, for the first time, we present a hydro-meteorological perspective based on five decades of synoptic weather observations collected by the meteorological network of Pakistan. Analysis of this unique data set indicates that increased regional scale humidity, cloud cover, and precipitation, along with decreased net radiation, near-surface wind speed, potential evapotranspiration and river flow, especially during the summer season, represent a substantial change in the energy, mass and momentum fluxes that are facilitating the establishment of the Karakoram Anomaly.PubDate: 2017-10-09T13:15:31.235188-05:DOI: 10.1002/2017GL075284

Authors:Gregory Cesana; Kay Suselj, Florent BrientAbstract: We investigate the effects of physical parameterizations on cloud feedback uncertainty in response to climate change. For this purpose, we construct an ensemble of eight aquaplanet simulations using the Weather Research and Forecasting (WRF) model. In each WRF-derived simulation, we replace only one parameterization at a time while all other parameters remain identical. By doing so, we aim to (i) reproduce cloud feedback uncertainty from state-of-the-art climate models and (ii) understand how parametrizations impact cloud feedbacks. Our results demonstrate that this ensemble of WRF simulations, which differ only in physical parameterizations, replicates the range of cloud feedback uncertainty found in state-of-the-art climate models. We show that microphysics and convective parameterizations govern the magnitude and sign of cloud feedbacks, mostly due to tropical low-level clouds in subsidence regimes. Finally, this study highlights the advantages of using WRF to analyze cloud feedback mechanisms owing to its plug-and-play parameterization capability.PubDate: 2017-10-09T13:10:26.362234-05:DOI: 10.1002/2017GL074820

Authors:Richard G. Williams; Vassil Roussenov, Thomas L. Frölicher, Philip GoodwinAbstract: The climate response after cessation of carbon emissions is examined here, exploiting a single equation connecting surface warming to cumulative carbon emissions. The multi-centennial response to an idealized pulse of carbon is considered by diagnosing a 1000 year integration of an Earth system model (GFDL ESM2M) and an ensemble of efficient Earth system model simulations. After emissions cease, surface temperature evolves according to (i) how much of the emitted carbon remains in the atmosphere and (ii) how much of the additional radiative forcing warms the surface rather than the ocean interior. The peak in surface temperature is delayed in time after carbon emissions cease through the decline in ocean heat uptake, which in turn increases the proportion of radiative forcing warming the surface. Eventually, after many centuries, surface temperature declines as the radiative forcing decreases through the excess atmospheric CO2 being taken up by the ocean and land.PubDate: 2017-10-09T13:01:14.02077-05:0DOI: 10.1002/2017GL075080

Authors:Wei Wang; Peter A. Cawood, Mei-Fu Zhou, Manoj K. Pandit, Xiao-Ping Xia, Jun-Hong ZhaoAbstract: The Malani Igneous Suite (MIS) in NW India represents one of the best preserved silicic large igneous provinces. Voluminous silicic lavas of the MIS erupted between ca. 780-750 Ma. Zircon grains from rhyolite and dacite lavas have oxygen isotopic compositions that include depleted (δ18O = 4.12 to -1.11‰) and enriched (δ18O = 8.23-5.12‰) signatures. The low-δ18O zircon grains have highly radiogenic Hf isotopic compositions (ƐHf(t)= +13.0 to +3.6), suggesting high temperature bulk cannibalization of upper level juvenile mafic crust as an essential mechanism to produce the low-δ18O felsic magma. Xenocrystic zircon grains in dacites have high δ18O and low ƐHf(t) values for magmas older than 800 Ma, reflecting a dramatic transition in tectono-thermal regime in NW India during 800-780 Ma. A synchronous transition also occurred in South China and Madagascar, suggesting a spatially linked geodynamic system. NW India and South China together with Madagascar and the Seychelles lay either along the periphery of Rodinia or outboards of the supercontinent with the age of convergent plate margin magmatism coinciding with breakup of the supercontinent.PubDate: 2017-10-09T12:30:24.89123-05:0DOI: 10.1002/2017GL074717

Authors:Minggang Xie; Meng-Hua Zhu, Zhiyong Xiao, Yunzhao Wu, Aoao XuAbstract: Whether or not background secondary craters dominate populations of small impact craters on terrestrial bodies is a half-century controversy. It has been suggested that small craters on some planetary bodies are dominated by background secondary craters based partly on the steepened slope of crater size-frequency distribution (CSFD) towards small diameters, such as the less than ~1 km diameter crater population on the lunar mare. Here we show that topography degradation enlarges craters and increases CSFD slopes with time. When topography degradation is taken into account, for various-aged crater populations, the observed steep CSFD at small diameters is uniformly consistent with an originally shallower CSFD, whose slope is undifferentiated from the CSFD slope estimated from near-Earth objects and terrestrial bolides. The results show that the effect of topography degradation on CSFD is important in dating planetary surfaces, and the steepening of CSFD slopes is not necessarily caused by secondary cratering, but rather a natural consequence of topography degradation.PubDate: 2017-10-06T17:20:35.447316-05:DOI: 10.1002/2017GL075298

Authors:Olivier Boucher; Christoph Kleinschmitt, Gunnar MyhreAbstract: Stratospheric sulfate aerosol injection (SAI) and marine cloud brightening (MCB) are the two most studied solar radiation management techniques. For the first time we combine them in a climate model to investigate their complementarity in terms of both instantaneous and effective radiative forcings. The effective radiative forcing induced by SAI is significantly stronger than its instantaneous counterpart evaluated at the top of atmosphere. Radiative kernel calculations indicate that this occurs because of a significant stratospheric warming and despite a large increase in stratospheric water vapor that strengthens the greenhouse effect. There is also a large decrease in high-level cloudiness induced by a stratification of the upper tropopause. Our model experiments also show that the radiative effects of SAI and MCB are quasi additive and have fairly complementary patterns in the Tropics. This results in less spatial and temporal variability in the radiative forcing for combined SAI and MCB as compared to MCB alone.PubDate: 2017-10-06T17:16:19.955279-05:DOI: 10.1002/2017GL074647

Authors:M. Deen; E. Wielandt, E. Stutzmann, W. Crawford, G. Barruol, K. SiglochAbstract: The Earth's hum is the permanent free oscillations of the Earth recorded in the absence of earthquakes, at periods above 30 seconds. We present the first observations of its fundamental spheroidal eigenmodes on broadband Ocean Bottom Seismometers (OBS) in the Indian Ocean. At the ocean bottom, the effects of ocean infragravity waves (compliance) and seafloor currents (tilt) overshadow the hum. In our experiment, data are also affected by electronic glitches. We remove these signals from the seismic trace by subtracting average glitch signals; performing a linear regression and using frequency-dependent response functions between pressure, horizontal and vertical seismic components. This reduces the long period noise on the OBS to the level of a good land station. Finally, by windowing the autocorrelation to include only the direct arrival, the first and second orbit around the Earth, and by calculating its Fourier transform, we clearly observe the eigenmodes at the ocean bottom.PubDate: 2017-10-06T17:16:07.127881-05:DOI: 10.1002/2017GL074892

Authors:N. Catalán; K. Bataille, R. ArayaAbstract: Using a non-homogeneous elastic model for the south Chile subduction zone, we calculate the depth-dependent geometry of the Liquiñe-Ofqui Fault Zone (LOFZ), considering that faults develop where shear stress is maximum. Shear stress develops due to the oblique subduction process, depending on shear modulus which varies as a function of the amount of fluids within the overriding plate. Regions with different values of shear modulus are obtained by the geometries of isotherms calculated from a thermal model. Based on the principle that fluids move from higher to lower pressure regions, we calculate paths of fluids from the subducting slab towards the free surface. In the vicinity of the volcanic arc, the obtained fluid paths agree with the geometry of the LOFZ, suggesting that margin-parallel strike-slip faults could serve as pathways for fluids through the overriding plate in oblique subduction zones.PubDate: 2017-10-02T17:40:37.875039-05:DOI: 10.1002/2017GL074870

Authors:Yuchen Wang; Kenji Satake, Takuto Maeda, Aditya Riadi GusmanPages: 10,282 - 10,289Abstract: We propose a new tsunami data assimilation approach based on Green's functions to reduce the computation time for tsunami early warning. Green's Function-based Tsunami Data Assimilation (GFTDA) forecasts the waveforms at points of interest (PoIs) by superposition of Green's functions between observation stations and PoIs. Unlike the previous assimilation approach, GFTDA does not require the calculation of the tsunami wavefield for the whole region during the assimilation process, because the Green's functions have been calculated in advance. The forecasted waveforms can be calculated by a simple matrix manipulation. The application to the tsunami waveforms recorded by the bottom pressure gauges of the Cascadia Initiative from the 2012 Haida Gwaii earthquake reveals that GFTDA achieves the same accuracy as the previous assimilation approach while reducing the time required to issue a valid tsunami warning.PubDate: 2017-10-28T19:25:36.770896-05:DOI: 10.1002/2017GL075307

Authors:Zhiyu Liu; Qiang Lian, Fangtao Zhang, Lei Wang, Mingming Li, Xiaolin Bai, Jianing Wang, Fan WangPages: 10,530 - 10,539Abstract: Despite its potential importance in the global climate system, mixing properties of the North Pacific low-latitude western boundary current system (LLWBC) remained unsampled until very recently. We report here on the first measurements of turbulence microstructure associated with these currents, made in the western boundary region of the tropical North Pacific east of the Philippines. The results suggest that thermocline mixing in the North Pacific LLWBC is generally weak with the diapycnal diffusivity κρ∼O(10−6) m2 s−1. This is consistent with predictions from internal wave-wave interaction theory that mixing due to internal wave breaking is significantly reduced at low latitudes. Enhanced mixing is found to be associated with a permanent cyclonic eddy, the Mindanao Eddy, but mainly at its south and north flanks. There, κρ is elevated by an order of magnitude due to eddy-induced geostrophic shear. Mixing in the eddy core is at the background level with no indication of enhancement.PubDate: 2017-10-28T19:15:59.787111-05:DOI: 10.1002/2017GL075210

Authors:F. Gasperini; M. E. Hagan, Y. ZhaoPages: 10,125 - 10,133Abstract: In the last decade evidence demonstrated that terrestrial weather greatly impacts the dynamics and mean state of the thermosphere via small-scale gravity waves and global-scale solar tidal propagation and dissipation effects. While observations have shown significant intraseasonal variability in the upper mesospheric mean winds, relatively little is known about this variability at satellite altitudes (∼250–400 km). Using cross-track wind measurements from the Challenging Minisatellite Payload and Gravity field and steady-state Ocean Circulation Explorer satellites, winds from a Modern-Era Retrospective Analysis for Research and Applications/Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model simulation, and outgoing longwave radiation data, we demonstrate the existence of a prominent and global-scale 90 day oscillation in the thermospheric zonal mean winds and in the diurnal eastward propagating tide with zonal wave number 3 (DE3) during 2009–2010 and present evidence of its connection to variability in tropospheric convective activity. This study suggests that strong coupling between the troposphere and the thermosphere occurs on intraseasonal timescales.PubDate: 2017-10-28T19:10:45.628141-05:DOI: 10.1002/2017GL075445

Authors:Sukyoung Lee; Tingting Gong, Steven B. Feldstein, James A. Screen, Ian SimmondsPages: 10,654 - 10,661Abstract: The Arctic has been warming faster than elsewhere, especially during the cold season. According to the leading theory, ice-albedo feedback warms the Arctic Ocean during the summer, and the heat gained by the ocean is released during the winter, causing the cold-season warming. Screen and Simmonds (2010; SS10) concluded that the theory is correct by comparing trend patterns in surface air temperature (SAT), surface turbulence heat flux (HF), and net surface infrared radiation (IR). However, in this comparison, downward IR is more appropriate to use. By analyzing the same data used in SS10 using the surface energy budget, it is shown here that over most of the Arctic the skin temperature trend, which closely resembles the SAT trend, is largely accounted for by the downward IR, not the HF, trend.PubDate: 2017-10-28T19:05:48.650838-05:DOI: 10.1002/2017GL075375

Authors:Peter F. Bernath; Mahdi Yousefi, Eric Buzan, Chris D. BoonePages: 10,735 - 10,743Abstract: The distributions of the four most abundant isotopologues and isotopomers (N2O, 15NNO, N15NO, and NN18O) of nitrous oxide have been measured in the Earth's stratosphere by infrared remote sensing with the Atmospheric Chemistry Experiment (ACE) Fourier transform spectrometer. These satellite observations have provided a near-global picture of N2O isotopic fractionation. The relative abundances of the heavier species increase with altitude and with latitude in the stratosphere as the air becomes older. The heavy isotopologues are enriched by 20–30% in the upper stratosphere and even more over the poles. These observations are in general agreement with model predictions made with the Whole Atmosphere Community Climate Model (WACCM). A detailed 3-D chemical transport model is needed to account for the global isotopic distributions of N2O and to infer sources and sinks.PubDate: 2017-10-28T19:00:59.578369-05:DOI: 10.1002/2017GL075122

Authors:Chao An; Chen Cai, Yong Zheng, Lingsen Meng, Philip LiuPages: 10,272 - 10,281Abstract: Seismic signals captured by ocean bottom pressure sensors, which are designed to record tsunami waves, are largely ignored. In this paper, we derive a simple theoretical solution of the ocean bottom pressure as a function of prescribed seafloor motion. All the assumptions are clearly stated and analyzed. The solution is checked by comparing the seafloor displacement and pressure from three M7+ earthquakes, recorded by ocean bottom seismometers and pressure gauges located off the Japanese coast. We then show two applications. First, using the seafloor displacement data recorded by an ocean bottom seismometer, the pressure amplitude recorded by the associated pressure gauge is corrected, and vice versa. Second, pressure recordings from Deep Ocean Assessment and Reporting of Tsunamis during the 2011 Tohoku earthquake are converted to seafloor displacements, which are then utilized to estimate the earthquake focal mechanism. Thus, we demonstrate that seismic signals recorded by pressure sensors have great potential for fast estimate of earthquake source parameters.PubDate: 2017-10-28T19:00:42.251891-05:DOI: 10.1002/2017GL075137

Authors:A. R. L. Kushnir; C. Martel, R. Champallier, F. B. WadsworthPages: 10,262 - 10,271Abstract: Heat from inflowing magma may act to seal permeable networks that assist passive outgassing at volcanic conduit margins and in overlying domes, reducing the efficiency of overpressure dissipation. Here we present a study of the evolution of permeability—measured under magmatic conditions—with increasing temperature in glassy and glass-poor basaltic andesites from Merapi volcano (Indonesia). Whereas the permeability of glass-poor rocks decreases little up to a temperature of 1,010°C, glassy specimens experience a pronounced decrease in permeability above the glass transition once the viscosity of the crystal suspension is low enough to relax under external stresses. Changes in temperature alone are thus not enough to significantly modify the permeability of the glass-poor rocks that commonly form Merapi's dome. However, the presence of glass-rich domains in a dome may lead to local sealing of the volcanic plumbing between eruptions, exacerbating localized overpressure development that could contribute to explosivity.PubDate: 2017-10-25T19:15:32.741427-05:DOI: 10.1002/2017GL074042

Authors:Wei Wang; Hui Lu, L. Ruby Leung, Hong-Yi Li, Jianshi Zhao, Fuqiang Tian, Kun Yang, Khem SotheaPages: 10,378 - 10,386Abstract: Water resources management, in particular flood control, in the Lancang-Mekong River Basin (LMRB) faces two key challenges in the 21st century: climate change and dam construction. A large-scale distributed Geomorphology-Based Hydrological Model coupled with a simple reservoir regulation model (GBHM-LMK-SOP) is used to investigate the relative effects of climate change and dam construction on the flood characteristics in the LMRB. Results suggest an increase in both flood magnitude and frequency under climate change, which is more severe in the upstream basin and increases over time. However, stream regulation by dam reduces flood risk consistently throughout this century, with more obvious effects in the upstream basin where larger reservoirs will be located. The flood mitigation effect of dam regulation dominates over the flood intensification effect of climate change before 2060, but the latter emerges more prominently after 2060 and dominates the flood risk especially in the lower basin.PubDate: 2017-10-25T19:10:45.497168-05:DOI: 10.1002/2017GL075037

Authors:Benjamin Seiyon Lee; Murali Haran, Klaus KellerPages: 10,617 - 10,623Abstract: Storm surges are key drivers of coastal flooding, which generate considerable risks. Strategies to manage these risks can hinge on the ability to (i) project the return periods of extreme storm surges and (ii) detect potential changes in their statistical properties. There are several lines of evidence linking rising global average temperatures and increasingly frequent extreme storm surges. This conclusion is, however, subject to considerable structural uncertainty. This leads to two main questions: What are projections under various plausible statistical models' How long would it take to distinguish among these plausible statistical models' We address these questions by analyzing observed and simulated storm surge data. We find that (1) there is a positive correlation between global mean temperature rise and increasing frequencies of extreme storm surges; (2) there is considerable uncertainty underlying the strength of this relationship; and (3) if the frequency of storm surges is increasing, this increase can be detected within a multidecadal timescale (≈20 years from now).PubDate: 2017-10-25T19:00:24.528208-05:DOI: 10.1002/2017GL074606

Authors:Cara C. Manning; Evan M. Howard, David P. Nicholson, Brenda Y. Ji, Zoe O. Sandwith, Rachel H. R. StanleyPages: 10,511 - 10,519Abstract: Measurement of the triple oxygen isotope (TOI) composition of O2 is an established method for quantifying gross oxygen production (GOP) in natural waters. A standard assumption to this method is that the isotopic composition of H2O, the substrate for photosynthetic O2, is equivalent to Vienna standard mean ocean water (VSMOW). We present and validate a method for estimating the TOI composition of H2O based on mixing of local meteoric water and seawater H2O end-members, and incorporating the TOI composition of H2O into GOP estimates. In the ocean, GOP estimates based on assuming the H2O is equivalent to VSMOW can have systematic errors of up to 48% and in low-salinity systems, errors can be a factor of 2 or greater. In future TOI-based GOP studies, TOI measurements of O2 and H2O should be paired when the H2O isotopic composition is expected to differ from VSMOW.PubDate: 2017-10-25T18:35:25.428261-05:DOI: 10.1002/2017GL074375

Authors:Andrew G. Pauling; Inga J. Smith, Patricia J. Langhorne, Cecilia M. BitzPages: 10,454 - 10,461Abstract: Earth System Models do not reproduce the observed increase in Antarctic sea ice extent which may be due to the unrealistic representation of ice shelves. Here we investigate the response of sea ice to increasing freshwater input from ice shelves using the Community Earth System Model with the Community Atmosphere Model version 5 [CESM1(CAM5)]. We have conducted model experiments adding fresh water as if from ice shelf melt with a linear increase in the rate of input over the period 1980–2013. Including the effect of heat loss from the ocean to melt ice shelves resulted in significantly more positive trends in sea ice area. We found that an increase in the rate of change of freshwater input of ∼45 Gt yr−2 was sufficient to offset the negative trend in sea ice area in CESM1(CAM5), although the freshwater input by the end of the experiment was larger than observed at that time.PubDate: 2017-10-25T18:25:31.218547-05:DOI: 10.1002/2017GL075017

Authors:Andrea Storto; Chunxue Yang, Simona MasinaPages: 10,520 - 10,529Abstract: The Earth's energy imbalance (EEI) is stored in the oceans for the most part. Thus, estimates of its variability can be ingested in ocean retrospective analyses to constrain the global ocean heat budget. Here we propose a scheme to assimilate top of the atmosphere global radiation imbalance estimates from Clouds and the Earth's Radiant Energy System (CERES) in a coarse-resolution variational ocean reanalysis system (2000–2014). The methodology proves able to shape the heat content tendencies according to the EEI estimates, without compromising the reanalysis accuracy. Spurious variability and underestimation (overestimation) present in experiments with in situ (no) data assimilation disappear when EEI data are assimilated. The warming hiatus present without the assimilation of EEI data is mitigated, inducing ocean warming at depths below 1,500 m and slightly larger in the Southern Hemisphere, in accordance with recent studies. Furthermore, the methodology may be applied to Earth System reanalyses and climate simulations to realistically constrain the global energy budget.PubDate: 2017-10-25T18:15:40.445479-05:DOI: 10.1002/2017GL075396

Authors:Homero Paltan; Duane Waliser, Wee Ho Lim, Bin Guan, Dai Yamazaki, Raghav Pant, Simon DadsonPages: 10,387 - 10,395Abstract: While emerging regional evidence shows that atmospheric rivers (ARs) can exert strong impacts on local water availability and flooding, their role in shaping global hydrological extremes has not yet been investigated. Here we quantify the relative contribution of ARs variability to both flood hazard and water availability. We find that globally, precipitation from ARs contributes 22% of total global runoff, with a number of regions reaching 50% or more. In areas where their influence is strongest, ARs may increase the occurrence of floods by 80%, while absence of ARs may increase the occurrence of hydrological droughts events by up to 90%. We also find that ~300 million people are exposed to additional floods and droughts due the occurrence of ARs. ARs provide a source of hydroclimatic variability whose beneficial or damaging effects depend on the capacity of water resources managers to predict and adapt to them.PubDate: 2017-10-25T18:10:45.72091-05:0DOI: 10.1002/2017GL074882

Authors:Laurie S. Huning; Steven A. Margulis, Bin Guan, Duane E. Waliser, Paul J. NeimanPages: 10,445 - 10,453Abstract: This study investigates the extent to which the diagnosed contribution of atmospheric rivers (ARs) to the seasonal cumulative snowfall (CS) is related to the AR detection approach utilized. Using both satellite integrated water vapor (IWV)-based and reanalysis integrated vapor transport (IVT)-based methodologies, the corresponding AR-derived CS distributions were characterized over the Sierra Nevada (USA) from 1998 to 2015. AR detection methods indicated that ARs yield greater orographic enhancement of the seasonal CS than non-AR storms above ~2,100–2,300 m for the IWV-based approach and over all elevations for the IVT-based detection approach across the western (i.e., windward) Sierra Nevada. Due to differences in the methodologies, the IWV-based approach diagnosed 2.1 times fewer ARs than the IVT-based approach. As a result, the ARs diagnosed using the IWV-based detection method yielded an average 33% of the total range-wide CS annually as opposed to 56% from the IVT-based detection method.PubDate: 2017-10-25T18:05:36.992907-05:DOI: 10.1002/2017GL075201

Authors:Christian Haas; Justin Beckers, Josh King, Arvids Silis, Julienne Stroeve, Jeremy Wilkinson, Bernice Notenboom, Axel Schweiger, Stefan HendricksPages: 10,462 - 10,469Abstract: In April 2017, we collected unique, extensive in situ data of sea ice and snow thickness. At 10 sampling sites, located under a CryoSat-2 overpass, between Ellesmere Island and 87.1°N mean and modal total ice thicknesses ranged between 2 to 3.4 m and 1.8 to 2.9 m, respectively. Coincident snow thicknesses ranged between 0.3 to 0.47 m (mean) and 0.1 to 0.5 m (mode). The profile spanned the complete multiyear ice zone in the Lincoln Sea, into the first-year ice zone farther north. Complementary snow thickness measurements near the North Pole showed a mean thickness of 0.31 m. Compared with scarce measurements from other years, multiyear ice was up to 0.75 m thinner than in 2004, but not significantly different from 2011 and 2014. We found excellent agreement with a commonly used snow climatology and with published long-term ice thinning rates. There was reasonable agreement with CryoSat-2 thickness retrievals.PubDate: 2017-10-25T18:00:33.008175-05:DOI: 10.1002/2017GL075434

Authors:C. M. Liu; H. S. Fu, J. B. Cao, Y. Xu, Y. Q. Yu, E. A. Kronberg, P. W. DalyPages: 10,116 - 10,124Abstract: The pitch angle distribution (PAD) of suprathermal electrons can have both spatial and temporal evolution in the magnetotail and theoretically can be an indication of electron energization/cooling processes there. So far, the spatial evolution of PAD has been well studied, leaving the temporal evolution as an open question. To reveal the temporal evolution of electron PAD, spacecraft should monitor the same flux tube for a relatively long period, which is not easy in the dynamic magnetotail. In this study, we present such an observation by Cluster spacecraft in the magnetotail behind a dipolarization front (DF). We find that the PAD of suprathermal electrons can evolve from pancake type to butterfly type duringPubDate: 2017-10-25T17:55:32.891022-05:DOI: 10.1002/2017GL075007

Authors:T. Kaku; J. Haruyama, W. Miyake, A. Kumamoto, K. Ishiyama, T. Nishibori, K. Yamamoto, Sarah T. Crites, T. Michikami, Y. Yokota, R. Sood, H. J. Melosh, L. Chappaz, K. C. HowellPages: 10,155 - 10,161Abstract: Intact lunar lava tubes offer a pristine environment to conduct scientific examination of the Moon's composition and potentially serve as secure shelters for humans and instruments. We investigated the SELENE Lunar Radar Sounder (LRS) data at locations close to the Marius Hills Hole (MHH), a skylight potentially leading to an intact lava tube, and found a distinctive echo pattern exhibiting a precipitous decrease in echo power, subsequently followed by a large second echo peak that may be evidence for the existence of a lava tube. The search area was further expanded to 13.00–15.00°N, 301.85–304.01°E around the MHH, and similar LRS echo patterns were observed at several locations. Most of the locations are in regions of underground mass deficit suggested by GRAIL gravity data analysis. Some of the observed echo patterns are along rille A, where the MHH was discovered, or on the southwest underground extension of the rille.PubDate: 2017-10-25T17:16:16.222886-05:DOI: 10.1002/2017GL074998

Authors:Konstantinos M. Andreadis; Guy J.-P. Schumann, Dimitrios Stampoulis, Paul D. Bates, G. Robert Brakenridge, Albert J. KettnerPages: 10,369 - 10,377Abstract: Floods are costly to global economies and can be exceptionally lethal. The ability to produce consistent flood hazard maps over large areas could provide a significant contribution to reducing such losses, as the lack of knowledge concerning flood risk is a major factor in the transformation of river floods into flood disasters. In order to accurately reproduce flooding in river channels and floodplains, high spatial resolution hydrodynamic models are needed. Despite being computationally expensive, recent advances have made their continental to global implementation feasible, although inputs for long-term simulations may require the use of reanalysis meteorological products especially in data-poor regions. We employ a coupled hydrologic/hydrodynamic model cascade forced by the 20CRv2 reanalysis data set and evaluate its ability to reproduce flood inundation area and volume for Australia during the 1973–2012 period. Ensemble simulations using the reanalysis data were performed to account for uncertainty in the meteorology and compared with a validated benchmark simulation. Results show that the reanalysis ensemble capture the inundated areas and volumes relatively well, with correlations for the ensemble mean of 0.82 and 0.85 for area and volume, respectively, although the meteorological ensemble spread propagates in large uncertainty of the simulated flood characteristics.PubDate: 2017-10-25T17:06:22.055508-05:DOI: 10.1002/2017GL075502

Authors:Luigi Cavaleri; Luciana BertottiPages: 10,504 - 10,510Abstract: Within the progressive improvement in wave modeling we focus on the attenuation of swell waves by rain. Till now ignored, the process is shown to be relevant, especially for the correct estimate of swell. Following the practical impossibility of devoted field experiments, we exploit the global model results over a period of more than four years to extract the tiny signal associated to the attenuation by rain. A direct comparison of the ratio model by altimeter significant wave heights versus the encountered rain amount hints to a marked related dependence. A proper quantification of the related physical effect requires a multiple step procedure that we describe in detail. We check the reliability of the results, and we provide the related source function ready for implementation in operational wave models.PubDate: 2017-10-24T17:45:27.4447-05:00DOI: 10.1002/2017GL075458

Authors:F. Catapano; G. Zimbardo, S. Perri, A. Greco, D. Delcourt, A. Retinò, I. J. CohenPages: 10,108 - 10,115Abstract: Energetic particles with energies from tens of keV to a few hundred keV are frequently observed in the Earth's magnetotail. Here we study, by means of a test particle numerical simulation, the acceleration of different ion species (H+, He+, He++, and On+ with n = 1–6) in the presence of transient electromagnetic perturbations. All the considered ions develop power law tails at high energies, except for O+ ions. This is strongly correlated to the time that the particle spends in the current sheet. Ion acceleration is found to be proportional to the charge state, while it grows in a weaker way with the ion mass. We find that O5+/6+ can reach energies higher than 500 keV. These results may explain the strong oxygen acceleration observed in the magnetotail.PubDate: 2017-10-24T17:40:27.091007-05:DOI: 10.1002/2017GL075092

Authors:Anne Valovcin; Toshiro TanimotoPages: 10,256 - 10,261Abstract: Tornadoes generate seismic signals when they contact the ground. Here we examine the signals excited by the Joplin tornado, which passed within 2 km of a station in the Earthscope Transportable Array. We model the tornado-generated vertical seismic signal at low frequencies (0.01–0.03 Hz) and solve for the strength of the seismic source. The resulting source amplitude is largest when the tornado was reported to be strongest (EF 4–5), and the amplitude is smallest when the tornado was weak (EF 0–2). A further understanding of the relationship between source amplitude and tornado intensity could open up new ways to study tornadoes from the ground.PubDate: 2017-10-24T09:00:03.193037-05:DOI: 10.1002/2017GL074185

Authors:Guangyu Fu; Yawen ShePages: 10,233 - 10,239Abstract: We built the first dense gravity network including 107 stations around the Tsangpo Gorge, Tibet, one of the hardest places in the world to reach, and conducted a gravity and hybrid GPS observation campaign in 2016. We computed the Bouguer gravity anomalies (BGAs) and free-air gravity anomalies (FGAs) and increased the resolution of the FGAs by merging the in situ data with EIGEN-6C4 gravity model data. The BGAs around the Tsangpo Gorge are in general negative and gradually decrease from south (−360 mGal) to north (−480 mGal). They indicate a uniformly dipping Moho around the Tsangpo Gorge that sinks from south to north at an angle of 12°. We introduced a method to compute the vertical tectonic stress of the lithosphere, a quantitative expression of isostasy, using BGA and terrain data, and applied it to the area around the Tsangpo Gorge. We found that the lithosphere of the upstream of the Tsangpo Gorge is roughly in an isostatic state, but the lithosphere of the downstream exhibits vertical tectonic stress of ~50 MPa, which indicates the loss of a large amount of surface material. This result does not support the deduction of the valley bottom before uplift of the Tsangpo Gorge by Wang et al. (2014).PubDate: 2017-10-21T19:45:38.501421-05:DOI: 10.1002/2017GL075290

Authors:Samuel N. Stechmann; Scott HottovyPages: 10,713 - 10,724Abstract: In the tropics, rainfall is coupled with atmospheric dynamics in ways that are not fully understood, and often different mechanisms are proposed to underlie different modes of variability. Here it is shown that a unified model with a simple form can produce many different modes of variability. In particular, this includes the Madden-Julian Oscillation and convectively coupled equatorial waves. The model predicts the length scales, time scales, structures, and spatiotemporal variability of these modes reasonably well for a simple model. Furthermore, the model produces a background spectrum of rainfall that resembles spatiotemporal red noise and is only weakly coupled with wave dynamics. The full spectrum is also shown to be shaped by antiresonance, whereby rainfall oscillations are prevented from occurring at the oscillation frequencies of dry waves. To produce all of these aspects simultaneously, a key factor is differing roles of lower and middle tropospheric water vapor.PubDate: 2017-10-21T19:35:49.109346-05:DOI: 10.1002/2017GL075754

Authors:C. Rutjes; G. Diniz, I. S. Ferreira, U. EbertPages: 10,702 - 10,712Abstract: Thunderstorms are known to create terrestrial gamma ray flashes (TGFs) which are microsecond-long bursts created by runaway of thermal electrons from propagating lightning leaders, as well as gamma ray glows that possibly are created by relativistic runaway electron avalanches (RREA) that can last for minutes or more and are sometimes terminated by a discharge. In this work we predict a new intermediate thunderstorm radiation mechanism, which we call TGF afterglow, as it is caused by the capture of photonuclear neutrons produced by a TGF. TGF afterglows are milliseconds to seconds long; this duration is caused by the thermalization time of the intermediate neutrons. TGF afterglows indicate that the primary TGF has produced photons in the energy range of 10–30 MeV; they are nondirectional in contrast to the primary TGF. Gurevich et al. might have reported TGF afterglows in 2011.PubDate: 2017-10-21T19:30:35.410336-05:DOI: 10.1002/2017GL075552

Authors:Sarah J. Perry; Shayne McGregor, Alex Sen Gupta, Matthew H. EnglandPages: 10,608 - 10,616Abstract: Potential changes to the El Niño–Southern Oscillation (ENSO) resulting from climate change may have far reaching impacts through atmospheric teleconnections. Here ENSO temperature and precipitation teleconnections between the historical and high-emission future simulations are compared in 40 models from phase 5 of the Coupled Model Intercomparison Project. Focusing on the global land area only, we show that there are robust increases in the spatial extent of ENSO teleconnections during austral summer in 2040–2089 of ~19% for temperature and ~12% for precipitation in the multimodel mean (MMM), relative to the 1950–1999 period. The MMM further shows that the expansion of ENSO teleconnection extent is at least partly related to a strengthening of ENSO teleconnections over continental regions; however, a consistent strengthening is not found across the individual models. This suggests that while more land may be affected by ENSO, the existing teleconnections may not be simply strengthened.PubDate: 2017-10-21T19:25:39.273377-05:DOI: 10.1002/2017GL074509

Authors:Zoltan B. Szuts; Christopher S. MeinenPages: 10,495 - 10,503Abstract: The Florida Current (FC) contributes to Atlantic circulation by carrying the western boundary flow of the subtropical gyre and the upper branch of meridional overturning circulation. Repeated FC hydrographic (velocity, salinity, and temperature) sections during 1982–1987 and 2001–2015 characterize its water mass structure and associated transport variability. On average, FC volume transport comes from subtropical North Atlantic water (NAW, 44%), Antarctic Intermediate Water (AAIW, 14%), surface water (SW, 27%), and an indistinct source (Rem 15%), while salinity transport relative to the average salinity along 26°N comes from NAW (55%), AAIW (0.2%), SW (30%), and Rem (15%). From 1982–1987 to 2001–2015, NAW, AAIW, and Rem salinified by 0.03–0.16 g kg−1 and increased the salinity anomaly transport by 3%. These patterns imply that advective salt transport by the FC (1) is sensitive to subtropical North Atlantic variability and (2) is partially decoupled from the volumetric pathway of the upper overturning branch.PubDate: 2017-10-21T19:20:29.255243-05:DOI: 10.1002/2017GL074538

Authors:Yan Xia; Yi HuangPages: 10,592 - 10,600Abstract: The tropical atmospheric circulation is projected to weaken during global warming, although the mechanisms that cause the weakening remain to be elucidated. We hypothesize that the weakening is related to the inhomogeneous distribution of the radiative forcing and feedback, which heats the tropical atmosphere in the ascending and subsiding regions differentially and thus requires the circulation to weaken due to energetic constraints. We test this hypothesis in a series of numerical experiments using a fully coupled general circulation model (GCM), in which the radiative forcing distribution is controlled using a novel method. The results affirm the effect of inhomogeneous forcing on the tropical circulation weakening, and this effect is greatly amplified by radiative feedback, especially that of clouds. In addition, we find that differential heating explains the intermodel differences in tropical circulation response to CO2 forcing in the GCM ensemble of the Climate Model Intercomparison Project.PubDate: 2017-10-21T19:15:53.336027-05:DOI: 10.1002/2017GL075678

Authors:Sumit ChakrabortyPages: 10,324 - 10,327Abstract: The equations governing the interaction of viscous deformation with porous flow of fluids give rise to wave-like solutions. Such solutions have been explored in the context of melt and fluid flow in the mantle and crust at high temperatures, where ductile behavior occurs. Now it has been shown that the coupling of the kinetics of chemical reactions with fluid flow may give rise to similar solutions. This opens intriguing new possibilities. Porosity waves may arise in low-temperature regimes, and may become more accessible to observation, or they may remain mathematical curiosities because other modes of transport dominate in such settings. A number of possibilities, questions, and future courses of research have been opened up by Omlin et al. (2017).PubDate: 2017-10-21T19:15:27.110164-05:DOI: 10.1002/2017GL075798

Authors:Sin-Mei Wu; Kevin M. Ward, Jamie Farrell, Fan-Chi Lin, Marianne Karplus, Robert B. SmithPages: 10,240 - 10,247Abstract: The Upper Geyser Basin in Yellowstone National Park contains one of the highest concentrations of hydrothermal features on Earth including the iconic Old Faithful geyser. Although this system has been the focus of many geological, geochemical, and geophysical studies for decades, the shallow (PubDate: 2017-10-21T19:10:52.335155-05:DOI: 10.1002/2017GL075255

Authors:Hamish McGowan; Alison TheobaldPages: 10,601 - 10,607Abstract: The most devastating mass coral bleaching has occurred during El Niño events, with bleaching reported to be a direct result of increased sea surface temperatures (SSTs). However, El Niño itself does not cause SSTs to rise in all regions that experience bleaching. Nor is the upper ocean warming trend of 0.11°C per decade since 1971, attributed to global warming, sufficient alone to exceed the thermal tolerance of corals. Here we show that weather patterns during El Niño that result in reduced cloud cover, higher than average air temperatures and higher than average atmospheric pressures, play a crucial role in determining the extent and location of coral bleaching on the world's largest coral reef system, the World Heritage Great Barrier Reef (GBR), Australia. Accordingly, synoptic-scale weather patterns and local atmosphere-ocean feedbacks related to El Niño–Southern Oscillation (ENSO) and not large-scale SST warming due to El Niño alone and/or global warming are often the cause of coral bleaching on the GBR.PubDate: 2017-10-21T18:50:39.718806-05:DOI: 10.1002/2017GL074877

Authors:Shaul Hurwitz; David R. ShellyPages: 10,328 - 10,331Abstract: Old Faithful geyser in Yellowstone National Park has attracted scientific research for almost a century and a half. Temperature and pressure measurements and video recordings in the geyser's conduit led to proposals of many quantitative eruption models. Nevertheless, information on the processes that initiate the geyser's eruption in the subsurface remained limited. Two new studies, specifically Wu et al. (2017) and Ward and Lin (2017), take advantage of recent developments in seismic data acquisition technology and processing methods to illuminate subsurface structures. Using a dense array of three-component nodal geophones, these studies delineate subsurface structures on a scale larger than previously realized, which exert control on the spectacular eruptions of Old Faithful geyser.PubDate: 2017-10-21T18:50:22.847921-05:DOI: 10.1002/2017GL075833

Authors:Haje Korth; Catherine L. Johnson, Lydia Philpott, Nikolai A. Tsyganenko, Brian J. AndersonPages: 10,147 - 10,154Abstract: Mercury's solar wind and interplanetary magnetic field environment is highly dynamic, and variations in these external conditions directly control the current systems and magnetic fields inside the planetary magnetosphere. We update our previous static model of Mercury's magnetic field by incorporating variations in the magnetospheric current systems, parameterized as functions of Mercury's heliocentric distance and magnetic activity. The new, dynamic model reproduces the location of the magnetopause current system as a function of systematic pressure variations encountered during Mercury's eccentric orbit, as well as the increase in the cross-tail current intensity with increasing magnetic activity. Despite the enhancements in the external field parameterization, the residuals between the observed and modeled magnetic field inside the magnetosphere indicate that the dynamic model achieves only a modest overall improvement over the previous static model. The spatial distribution of the residuals in the magnetic field components shows substantial improvement of the model accuracy near the dayside magnetopause. Elsewhere, the large-scale distribution of the residuals is similar to those of the static model. This result implies either that magnetic activity varies much faster than can be determined from the spacecraft's passage through the magnetosphere or that the residual fields are due to additional external current systems not represented in the model or both. Birkeland currents flowing along magnetic field lines between the magnetosphere and planetary high-latitude regions have been identified as one such contribution.PubDate: 2017-10-21T18:35:33.997956-05:DOI: 10.1002/2017GL074699

Authors:Augusto Getirana; Sujay Kumar, Manuela Girotto, Matthew RodellPages: 10,359 - 10,368Abstract: This study quantifies the contribution of rivers and floodplains to terrestrial water storage (TWS) variability. We use state-of-the-art models to simulate land surface processes and river dynamics and to separate TWS into its main components. Based on a proposed impact index, we show that surface water storage (SWS) contributes 8% of TWS variability globally, but that contribution differs widely among climate zones. Changes in SWS are a principal component of TWS variability in the tropics, where major rivers flow over arid regions and at high latitudes. SWS accounts for ~22–27% of TWS variability in both the Amazon and Nile Basins. Changes in SWS are negligible in the Western U.S., Northern Africa, Middle East, and central Asia. Based on comparisons with Gravity Recovery and Climate Experiment-based TWS, we conclude that accounting for SWS improves simulated TWS in most of South America, Africa, and Southern Asia, confirming that SWS is a key component of TWS variability.PubDate: 2017-10-21T18:30:43.198456-05:DOI: 10.1002/2017GL074684

Authors:Pietro Milillo; Eric Rignot, Jeremie Mouginot, Bernd Scheuchl, Mathieu Morlighem, Xin Li, Jacqueline T. SalzerPages: 10,436 - 10,444Abstract: Using radar satellite data from the Italian COSMO-SkyMed (CSK) constellation and the German TanDEM-X formation, we present comprehensive measurements of the biweekly grounding line dynamics of Pine Island Glacier, West Antarctica, from August to December 2015. The 1 day repeat cycle of CSK reveals tidally induced, grounding line migration on the scale of kilometers and extensive seawater intrusion within the grounding zone, which significantly exceeds that predicted for a stiff bed but are consistent with that calculated for a deformable bed. The deformable bed also explains the continuous draining/filling of subglacial lakes proximal to the grounding line. After correction for oceanic tides, we estimate a retreat rate for 2011–2015 of 0.3 km/yr at the glacier center and 0.5 km/yr on the sides, which is 3 times slower than for 1994–2011 (1.2 km/yr at the center). We attribute the decrease in retreat rate to colder ocean conditions in 2012–2013 relative to 2000–2011.PubDate: 2017-10-21T18:25:56.089327-05:DOI: 10.1002/2017GL074320

Authors:Chunquan Yu; Zhongwen Zhan, Egill Hauksson, Elizabeth S. CochranPages: 10,208 - 10,215Abstract: Seismic scattering is commonly observed and results from wave propagation in heterogeneous medium. Yet deterministic characterization of scatterers associated with lateral heterogeneities remains challenging. In this study, we analyze broadband waveforms recorded by the Southern California Seismic Network and observe strongly scattered Love waves following the arrival of teleseismic SH wave. These scattered Love waves travel approximately in the same (azimuthal) direction as the incident SH wave at a dominant period of ~10 s but at an apparent velocity of ~3.6 km/s as compared to the ~11 km/s for the SH wave. Back projection suggests that this strong scattering is associated with pronounced bathymetric relief in the Southern California Continental Borderland, in particular the Patton Escarpment. Finite-difference simulations using a simplified 2-D bathymetric and crustal model are able to predict the arrival times and amplitudes of major scatterers. The modeling suggests a relatively low shear wave velocity in the Continental Borderland.PubDate: 2017-10-21T18:20:43.943126-05:DOI: 10.1002/2017GL075213

Authors:Justin Yen-Ting Ko; Don V. Helmberger, Huilin Wang, Zhongwen ZhanPages: 10,216 - 10,225Abstract: The morphologies of subducted remnants in the lower mantle are essential to our understanding of the history of plate tectonism. Here we image a high-velocity slab-like (HVSL) anomaly beneath the southeastern U.S. using waveforms from five deep earthquakes beneath South America recorded by the USArray. In addition to travel time anomalies, the multipathing of S and ScS phases at different distances are used to constrain the HVSL model. We jointly invert S and ScS traveltimes, amplitudes, and waveform complexities to produce a best fitting block model characterized by a rectangular shape with a 2.5% S wave velocity increase and tapered edges. While the Farallon slab is expected to dip primarily eastward, the HVSL structure apparently dips 40° to 50° to the SE and appears to be related to the eclogitized Hess conjugate.PubDate: 2017-10-21T18:15:48.115348-05:DOI: 10.1002/2017GL075032

Authors:Joeri Rogelj; Carl-Friedrich Schleussner, William HarePages: 10,662 - 10,665Abstract: The adoption of the 1.5°C long-term warming limit in the Paris Agreement made 1.5°C a “hot topic” in the scientific community, with researchers eager to address this issue. Long-term warming limits have a decade-long history in international policy. To effectively inform the climate policy debate, geoscience research hence needs a core understanding of their legal and policy context. Here we describe this context in detail and illustrate its importance by showing the impact it can have on global carbon budget estimates. We show that definitional clarity is essential on this important matter.PubDate: 2017-10-21T18:10:24.686636-05:DOI: 10.1002/2017GL075612

Authors:K. E. Knowland; L. E. Ott, B. N. Duncan, K. WarganPages: 10,691 - 10,701Abstract: Stratospheric intrusions have been the interest of decades of research for their ability to bring stratospheric ozone (O3) into the troposphere with the potential to enhance surface O3 concentrations. However, these intrusions have been misrepresented in models and reanalyses until recently, as the features of a stratospheric intrusion are best identified in horizontal resolutions of 50 km or smaller. NASA's Modern-Era Retrospective Analysis for Research and Applications Version-2 (MERRA-2) reanalysis is a publicly available high-resolution data set (∼50 km) with assimilated O3 that characterizes O3 on the same spatiotemporal resolution as the meteorology. We demonstrate the science capabilities of the MERRA-2 reanalysis when applied to the evaluation of stratospheric intrusions that impact surface air quality. This is demonstrated through a case study analysis of stratospheric intrusion-influenced O3 exceedances in spring 2012 in Colorado, using a combination of observations, the MERRA-2 reanalysis and Goddard Earth Observing System Model, Version 5 simulations.PubDate: 2017-10-21T18:01:01.442478-05:DOI: 10.1002/2017GL074532

Authors:Hongzhan Fei; Rong Huang, Xiaozhi YangPages: 10,226 - 10,232Abstract: The electrical conductivity of CaSiO3 perovskite was measured in situ between 17–24 GPa and 1300–2000 K using a multianvil apparatus and Solartron 1260 Impedance/Gain-Phase Analyzer in the frequency range of 107–1 Hz. The activation energies are 95–100 and 100–120 kJ/mol, and the activation volumes are 0.06 ± 0.08 and −0.46 ± 0.03 cm3/mol, at 1300–1800 and 1800–2000 K, respectively. Conduction under lower mantle conditions may be dominated by the ionic diffusion of oxygen. The electrical conductivity of CaSiO3 perovskite is higher than that of bridgmanite, majoritic garnet, and ferropericlase, the main constituents of the topmost lower mantle. Therefore, CaSiO3-perovsktie may significantly contribute to the electrical structure of the topmost lower mantle in spite of its relatively small volume proportion.PubDate: 2017-10-21T17:55:27.609045-05:DOI: 10.1002/2017GL075070

Authors:Katarina Miljković; Myriam Lemelin, Paul G. LuceyPages: 10,140 - 10,146Abstract: Numerical modeling of the peak-ring basin formation showed that the peak-ring forms from the material that is part of the central uplift outwardly thrust over the inwardly collapsing transient crater rim. Simulations of the lunar basin formation showed that the peak or inner ring in peak ring or multiring basins, respectively, is composed of the overturned crust and deep-seated material, possibly from the upper mantle. Numerical impact simulations were used to trace the depth of origin of material exposed within the peak (or inner) ring. We estimate the scaling trends between basin size and the depth of origin of material exposed within the ring. We also report on the likely crust, mantle, and projectile abundances exposed within the ring. Quantifying the excavation depths during the formation of the peak or inner ring provides a step toward understanding the lunar crust and mantle stratigraphy.PubDate: 2017-10-19T11:30:31.057874-05:DOI: 10.1002/2017GL075207

Authors:T. Tamarin; Y. KaspiPages: 10,666 - 10,674Abstract: Comprehensive models of climate change projections have shown that the latitudinal band of extratropical storms will likely shift poleward under global warming. Here we study this poleward shift from a Lagrangian storm perspective, through simulations with an idealized general circulation model. By employing a feature tracking technique to identify the storms, we demonstrate that the poleward motion of individual cyclones increases with increasing global mean temperature. A potential vorticity tendency analysis of the cyclone composites highlights two leading mechanisms responsible for enhanced poleward motion: nonlinear horizontal advection and diabatic heating associated with latent heat release. Our results imply that for a 4 K rise in the global mean surface temperature, the mean poleward displacement of cyclones increases by about 0.85° of latitude, and this occurs in addition to a poleward shift of about 0.6° in their mean genesis latitude. Changes in cyclone tracks may have a significant impact on midlatitude climate, especially in localized storm tracks such as the Atlantic and Pacific storm tracks, which may exhibit a more poleward deflected shape.PubDate: 2017-10-18T12:35:38.0876-05:00DOI: 10.1002/2017GL073633

Authors:Liying Qian; Alan Burns, Jia YuePages: 10,100 - 10,107Abstract: Numerical studies have shown that there is a lower thermospheric winter-to-summer circulation that is driven by wave dissipation and that it plays a significant role in trace gas distributions in the mesosphere and lower thermosphere, and in the composition of the thermosphere. However, the characteristics of this circulation are poorly known. Direct observations of it are difficult, but it leaves clear signatures in tracer distributions. The Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite has obtained CO2 concentration from 2002 to present. This data set, combined with simulations by the Whole Atmosphere Community Climate Model, provides an unprecedented opportunity to infer the morphology of this circulation in both the summer and winter hemispheres. Our study show that there exists a maximum vertical gradient of CO2 at summer high latitudes, driven by the convergence of the upwelling of the mesospheric circulation and the downwelling of the lower thermospheric circulation; in the winter hemisphere, the maximum vertical gradient of CO2 is located at a higher altitude, driven by the convergence of the upwelling of the lower thermospheric circulation and the downwelling of the solar-driven thermospheric circulation; the bottom of the lower thermospheric circulation is located between ~ 95 km and 100 km, and it has a vertical extent of ~10 km. Analysis of the SABER CO2 and temperature at summer high latitudes showed that the bottom of this circulation is consistently higher than the mesopause height by ~10 km.PubDate: 2017-10-18T05:40:38.528799-05:DOI: 10.1002/2017GL075643

Authors:Qiuyu Wang; Shuang Yi, Le Chang, Wenke SunPages: 10,427 - 10,435Abstract: Recently, increased efforts have been made to estimate the mass budgets of glaciers in High Mountain Asia (HMA). However, seasonal changes in glaciers are poorly understood, despite the fact that seasonal meltwater released from glaciers is a crucial local water resource in HMA. Utilizing satellite altimetry and gravimetry data, we constructed annual changes in glacier elevation and identified two general patterns of the seasonality of glacier elevation changes. Glaciers in the periphery of HMA (except for those in the eastern Himalayas) thicken from approximately December to April–June, thus exhibiting winter and spring accumulation. Glaciers in the inner Tibetan Plateau, especially those in Western Kunlun and Tanggula, accumulate from approximately March to approximately August, thus exhibiting spring and summer accumulation. The amounts of seasonal glacier ablation were obtained using a new approach of direct observations of glacier changes, rather than inferring changes using a climate model.PubDate: 2017-10-18T05:31:16.917203-05:DOI: 10.1002/2017GL075300

Authors:D. D. Parrish; I. Petropavlovskikh, S. J. OltmansPages: 10,675 - 10,681Abstract: Changes in baseline (here understood as representative of continental to hemispheric scales) tropospheric ozone concentrations that have occurred over western North American and eastern North Pacific are analyzed based on data from three measurement records: (1) sites in the U.S. Pacific coast marine boundary layer, (2) an inland, higher altitude site at Lassen Volcanic National Park, CA, and (3) springtime airborne measurements in the free troposphere between 3 and 8 km altitude. Consistent with previously published results, we find increasing ozone prior to the year 2000, but that rate of increase has slowed and now reversed in these data sets in all seasons. The past ozone increase has been identified as a significant difficulty to overcome in achieving U.S. air quality goals; this difficulty has now eased. Global models only poorly reproduce the observed baseline ozone and trends; policy guidance from such models must be considered very cautiously.PubDate: 2017-10-18T05:30:31.259472-05:DOI: 10.1002/2017GL074960

Authors:E. Spanswick; E. Donovan, L. Kepko, V. AngelopoulosPages: 10,094 - 10,099Abstract: The bright proton aurora is generally understood to be the projection of the Central Plasma Sheet where there is sufficient particle energy to cause auroral luminosity and strong pitch angle scattering (presumably due to field line curvature). This region is often interpreted as the transition region between dipolar and tail-like magnetic topologies. The location of auroral features relative to the peak in the proton aurora has been used, for example, to argue that the substorm onset arc lies on field lines that thread this transition region. In this paper, we present statistics of proton aurora luminosity computed from Time History of Events and Macroscale Interactions in Substorms electrostatic analyzer measurements for various radial distances in the magnetotail. These results are compared to ground observations of proton auroral luminosity and used to derive a statistical source region of the bright proton aurora.PubDate: 2017-10-18T05:11:11.066033-05:DOI: 10.1002/2017GL074956

Authors:Evan Weller; Christian Jakob, Michael J. ReederPages: 10,682 - 10,690Abstract: The parameterization of convection in climate models is a large source of uncertainty in projecting future precipitation changes. Here an objective method to identify organized low-level convergence lines has been used to better understand how atmospheric convection is organized and projected to change, as low-level convergence plays an important role in the processes leading to precipitation. The frequency and strength of convergence lines over both ocean and land in current climate simulations is too low compared to reanalysis data. Projections show a further reduction in the frequency and strength of convergence lines over the midlatitudes. In the tropics, the largest changes in frequency are generally associated with shifts in major low-latitude convergence zones, consistent with changes in the precipitation. Further, examining convergence lines when in the presence or absence of precipitation results in large spatial contrasts, providing a better understanding of regional changes in terms of thermodynamic and dynamic effects.PubDate: 2017-10-18T05:01:36.906975-05:DOI: 10.1002/2017GL075489

Authors:O. A. Tuinenburg; J. P. R. VriesPages: 10,341 - 10,348Abstract: Irrigation modulates the water cycle by making water available for plants, increasing transpiration and atmospheric humidity, while decreasing temperatures due to the energy that is needed for evaporation. Irrigation is usually not included in atmospheric reanalysis systems, but moisture can be added to the soil due to data assimilation. This paper compares these soil moisture additions to the irrigation patterns. In the ERA-interim atmospheric reanalysis, 2 m temperature observations are assimilated. A mismatch between modeled and observed temperatures is corrected by adding or removing moisture from the soil. These corrections show a clear pattern of mean soil moisture additions in many areas. To determine the cause of these increments, the spatial and temporal patterns of these soil moisture increments are compared to irrigation water demand and precipitation bias. In irrigated areas, the annual means and cycles of soil moisture increments correlate well with irrigation, and less with precipitation bias. Therefore, in irrigated areas, the soil moisture increments are more likely caused by irrigation than by the precipitation bias. In nonirrigated areas, a weak statistical relation between soil moisture increments and precipitation bias is present. Irrigation is currently not included in reanalysis systems. However, as irrigation indirectly influences the water balance in atmospheric reanalysis systems, we recommend to include this process in reanalysis models. Moreover, the influence of irrigation on the local and regional atmosphere should be taken into account when interpreting atmospheric data over strongly irrigated areas.PubDate: 2017-10-18T04:26:11.045641-05:DOI: 10.1002/2017GL074884

Authors:Laurel G. Larsen; Jie Ma, David KaplanPages: 10,349 - 10,358Abstract: How important is hydrologic connectivity for surface water fluxes through heterogeneous floodplains, deltas, and wetlands' While significant for management, this question remains poorly addressed. Here we adopt spatial resistance averaging, based on channel and patch configuration metrics quantifiable from aerial imagery, to produce an upscaled rate law for discharge. Our model suggests that patch coverage largely controls discharge sensitivity, with smaller effects from channel connectivity and vegetation patch fractal dimension. However, connectivity and patch configuration become increasingly important near the percolation threshold and at low water levels. These effects can establish positive feedbacks responsible for substantial flow change in evolving landscapes (14–36%, in our Everglades case study). Connectivity also interacts with other drivers; flow through poorly connected hydroscapes is less resilient to perturbations in other drivers. Finally, we found that flow through heterogeneous patches is alone sufficient to produce non-Manning flow–depth relationships commonly observed in wetlands but previously attributed to depth-varying roughness.PubDate: 2017-10-18T04:20:35.097167-05:DOI: 10.1002/2017GL075432

Authors:H. A. Singh; P. J. Rasch, B. E. J. RosePages: 10,583 - 10,591Abstract: We isolate the role of the ocean in polar climate change by directly evaluating how changes in ocean dynamics with quasi-equilibrium CO2 doubling impact high-latitude climate. With CO2 doubling, the ocean heat flux convergence (OHFC) shifts poleward in winter in both hemispheres. Imposing this pattern of perturbed OHFC in a global climate model results in a poleward shift in ocean-to-atmosphere turbulent heat fluxes (both sensible and latent) and sea ice retreat; the high latitudes warm, while the midlatitudes cool, thereby amplifying polar warming. Furthermore, midlatitude cooling is propagated to the polar midtroposphere on isentropic surfaces, augmenting the (positive) lapse rate feedback at high latitudes. These results highlight the key role played by the partitioning of meridional energy transport changes between the atmosphere and ocean in high-latitude climate change.PubDate: 2017-10-18T04:16:10.483037-05:DOI: 10.1002/2017GL074561

Authors:T. J. Aubry; G. Carazzo, A. M. JellinekPages: 10,198 - 10,207Abstract: Predictions for the heights and downwind trajectories of volcanic plumes using integral models are critical for the assessment of risks and climate impacts of explosive eruptions but are strongly influenced by parameterizations for turbulent entrainment. We compare four popular parameterizations using small scale laboratory experiments spanning the large range of dynamical regimes in which volcanic eruptions occur. We reduce uncertainties on the wind entrainment coefficient β which quantifies the contribution of wind-driven radial velocity shear to entrainment and is a major source of uncertainty for predicting plume height. We show that models better predict plume trajectories if (i) β is constant or increases with the plume buoyancy to momentum flux ratio and (ii) the superposition of the axial and radial velocity shear contributions to the turbulent entrainment is quadratic rather than linear. Our results have important implications for predicting the heights and likelihood of collapse of volcanic columns.PubDate: 2017-10-17T01:15:55.284786-05:DOI: 10.1002/2017GL075069

Authors:M. Elliot Smith; Noah J. Finnegan, Erich R. Mueller, Rebecca J. BestPages: 10,332 - 10,340Abstract: Though submarine canyons are first-order topographic features of Earth, the processes responsible for their occurrence remain poorly understood. Potentially analogous studies of terrestrial rivers show that the flux and caliber of transported bedload are significant controls on bedrock incision. Here we hypothesize that coarse sediment load could exert a similar role in the formation of submarine canyons. We conducted a comprehensive empirical analysis of canyon occurrence along the West Coast of the contiguous United States which indicates that submarine canyon occurrence is best predicted by the occurrence of durable crystalline bedrock in adjacent terrestrial catchments. Canyon occurrence is also predicted by the flux of bed sediment to shore from terrestrial streams. Surprisingly, no significant correlation was observed between canyon occurrence and the slope or width of the continental shelf. These findings suggest that canyon incision is promoted by greater yields of durable terrestrial clasts to the shore.PubDate: 2017-10-17T01:00:39.383359-05:DOI: 10.1002/2017GL075139

Authors:Daniel F. Schmidt; Kevin M. GrisePages: 10,573 - 10,582Abstract: Numerous lines of observational evidence suggest that Earth's tropical belt has expanded over the past 30–40 years. It is natural to expect that this poleward displacement should be associated with drying on the poleward margins of the subtropics, but it is less clear to what degree the drying should be zonally symmetric. This study tests the degree to which poleward motion of the Hadley cell boundary is associated with changes in local precipitation or sea level pressure and the degree to which those changes are zonally symmetric. Evidence from both reanalysis data and global climate models reveals that the local changes associated with Hadley cell expansion are mostly confined to certain centers of action which lie primarily over oceans. Consequently, the tropical expansion measured by zonally averaged variables is not associated with systematic drying over subtropical land regions, as is often assumed.PubDate: 2017-10-17T00:56:52.268155-05:DOI: 10.1002/2017GL075380

Authors:Vishnu Nandan; Torsten Geldsetzer, John Yackel, Mallik Mahmud, Randall Scharien, Stephen Howell, Joshua King, Robert Ricker, Brent ElsePages: 10,419 - 10,426Abstract: The European Space Agency's CryoSat-2 satellite mission provides radar altimeter data that are used to derive estimates of sea ice thickness and volume. These data are crucial to understanding recent variability and changes in Arctic sea ice. Sea ice thickness retrievals at the CryoSat-2 frequency require accurate measurements of sea ice freeboard, assumed to be attainable when the main radar scattering horizon is at the snow/sea ice interface. Using an extensive snow thermophysical property dataset from late winter conditions in the Canadian Arctic, we examine the role of saline snow on first-year sea ice (FYI), with respect to its effect on the location of the main radar scattering horizon, its ability to decrease radar penetration depth, and its impact on FYI thickness estimates. Based on the dielectric properties of saline snow commonly found on FYI, we quantify the vertical shift in the main scattering horizon. This is found to be approximately 0.07 m. We propose a thickness-dependent snow salinity correction factor for FYI freeboard estimates. This significantly reduces CryoSat-2 FYI retrieval error. Relative error reductions of ~11% are found for an ice thickness of 0.95 m and ~25% for 0.7 m. Our method also helps to close the uncertainty gap between SMOS and CryoSat-2 thin ice thickness retrievals. Our results indicate that snow salinity should be considered for FYI freeboard estimates.PubDate: 2017-10-17T00:50:34.091621-05:DOI: 10.1002/2017GL074506

Authors:P. Swapna; J. Jyoti, R. Krishnan, N. Sandeep, S. M. GriffiesPages: 10,560 - 10,572Abstract: North Indian Ocean sea level has shown significant increase during last three to four decades. Analyses of long-term climate data sets and ocean model sensitivity experiments identify a mechanism for multidecadal sea level variability relative to global mean. Our results indicate that North Indian Ocean sea level rise is accompanied by a weakening summer monsoon circulation. Given that Indian Ocean meridional heat transport is primarily regulated by the annual cycle of monsoon winds, weakening of summer monsoon circulation has resulted in reduced upwelling off Arabia and Somalia and decreased southward heat transport, and corresponding increase of heat storage in the North Indian Ocean. These changes in turn lead to increased retention of heat and increased thermosteric sea level rise in the North Indian Ocean, especially in the Arabian Sea. These findings imply that rising North Indian Ocean sea level due to weakening of monsoon circulation demands adaptive strategies to enable a resilient South Asian population.PubDate: 2017-10-17T00:15:35.873711-05:DOI: 10.1002/2017GL074706

Authors:Nicholas J. Mancinelli; Karen M. Fischer, Colleen A. DaltonPages: 10,189 - 10,197Abstract: Earth's cratonic mantle lithosphere is distinguished by high seismic wave velocities that extend to depths greater than 200 km, but recent studies disagree on the magnitude and depth extent of the velocity gradient at their lower boundary. Here we analyze and model the frequency dependence of Sp waves to constrain the lithosphere-asthenosphere velocity gradient at long-lived stations on cratons in North America, Africa, Australia, and Eurasia. Beneath 33 of 44 stations, negative velocity gradients at depths greater than 150 km are less than a 2–3% velocity drop distributed over more than 80 km. In these regions the base of the typical cratonic lithosphere is gradual enough to be explained by a thermal transition. Vertically sharper lithosphere-asthenosphere transitions are permitted beneath 11 stations, but these zones are spatially intermittent. These results demonstrate that lithosphere-asthenosphere viscosity contrasts and coupling fundamentally differ between cratons and younger continents.PubDate: 2017-10-17T00:05:46.186266-05:DOI: 10.1002/2017GL074518